So there's a lot to unpack here to I will try to take things argument-by-argument. Overall I think there is a real problem with this argument in that it conflates human-like intelligence and social structure with an ethical good and assumes a human-like existence is the end goal of all life on Earth, despite many, many lines of evidence to the contrary.
It should not be overlooked that the reports cited leaves much room for subtleties such as in-group versus out-group violence and the complexities of the motives. For instance humans display less in-group violence than chimps while achieving stupendous levels of out-group carnage. The take away here is that: 1) humanity isn't alone in organizing a group of own-species individuals to do deadly violence on another group of the same species. And 2) there are evolutionary pressures involved in this behavior, that is, greater social interaction is a driver for greater brain development.
Social complexity in general has been linked to violent behavior in general. A survey of intraspecific violence across mammals found that social structure was the greatest predictor of whether animals were likely to kill one another (e.g., colonial ground squirrels, social mongooses, and primates all had the highest rates of violence), and this is notably not tied to brain size/intelligence (ground squirrels have the smallest brain size of all rodents but are simultaneously the most violent. However, given that most intelligent animals tend to be social, most intelligent animals tend to be disproportionately violent.
Humans are kind of weird in that our rather bizarre social behavior has been suggested to be due to a combination of self-domestication as well as selection for a social system that doesn't result in us all killing each other. Namely we live at much higher densities than other primates, so high levels of aggression are maladaptive, yet we need some level of aggression in order for the social group to be able to protect itself from predators and competitors. The suggested end result is that we evolved a mental system of "mob violence" in which we're normally pretty placid but put under the right set of stimuli and perceived threat it can cause the whole group to go into a murderous rage (e.g., human warfare, witchhunts, etc.). This has resulted in us actually being abnormally placid for a primate, even moreso than bonobos believe it or not. This isn't even getting into how some of humanity's placidity is thought to be a result of autocrats and despots selectively culling more assertive or independent individuals through capital punishment, resulting in a population of mostly docile sheeple (as the autocrats in turn contributed little to the overall genepool due to population size), much as humans selectively killed off the most aggressive horses, dogs, cattle, etc.
However, this kind of gets into the question of the ethics you are proposing. What is "ethical" in terms of evolution? The implication here is that evolving a more human-like social system and increased sociality is "good", but there is equal evidence to suggest that increasing sociality results in more carnage and violence rather than the opposite.
So now we come to the 'ethics' part of this question. Humanity has set itself to the preservation of "Nature", conserving as much of species that politics and economics would allow. Yet the human condition is proof that existence is more that mere survival, it is Art, and Beauty, and transcendence... Does it not follow that we should afford all species every opportunity to evolve to the best of their potential?
Citation needed: How is the human existence proof that existence is about art, beauty, and transcendence? Many philosophers have concluded the exact opposite: that human existence is about nihlistic misery and we're merely the only animals smart enough to see the bars of our cage (e.g., Sophocles, this was in many ways the predominant viewpoint in many human cultures like the Greeks and Babylonians until they developed enough long-term economic prosperity). Given the lack of universal or even broad agreement on the topic and the fact that this definition of value seems to be mostly subjective, it's hard to see how it can be used to argue this point.
Similarly, what is "best of their potential"? OP seems to be arguing that all species should be naturally evolving towards full sapience. But evolution does not work this way, evolution selects for what is best right now, and what is "best" is what promotes the propagation and survival of specific phenotypes in the environment. Indeed, sapience is not a universal positive, there are at least two cases in mammals (myomorph rodents and opossums) in which smaller-brained species had a greater selective advantage over larger-brained ones, resulting in a net decrease in brain size over geological time. Similarly, there are many independent cases of species "degenerating" upon adoption of a parasitic lifestyle, losing complex organs such as eyes, brains, and digestive tracts in favor of a simple worm or tapeworm-like existence. Sacculina is a good example of this: despite being a mostly amorphic blob of flesh this animal is actually a type of barnacle, which in turn is a sedentary form of arthropod, so at some point the ancestors of Sacculina were free-living animals similar to a lobster or crayfish.
Now to make our failure plain through the case study of chimpanzees: we have given them space to live, to survive, but have we given them space to fight? To learn cooperation among themselves? To war? To learn cooperation amongst communities? To learn peace?
So it's true that human-esque thought processes and values are a lot more common among animals than most people like to admit (source: work with a bunch of behavioral ethologists that study this kind of thing), even if they aren't as complex as expressed in humans, but this kind of gets into a bit of anthropomorphism in that many species are characterized by very different temperaments due to social systems, physiological differences in hormones, etc. All species have similar values in the desire to find food, reproduce, etc., but their priorities and how they express them can be very different.
A great case study of this is the five great apes, who despite sharing a common ancestor < 10 million years ago all have very different temperaments. Human are unusual in our almost bipolar stance on violence (oscillating between periods of extreme violence and extreme peace depending on environmental contexts that aren't present in other sophonts like chimps, elephants, or crows), gorillas are notably calm, orangutans have a reputation as mischief-makers, bonobos are weirdly naïve, and chimps...I've heard chimps described as an entire species of low-grade sociopaths. They actually appear to have a sense of sadism, not merely harming animals (and their keepers) because they find toying with them amusing, but there are reports from workers in facilities with sign language-using chimpanzees that the chimps understand that their actions are harming others and find enjoyment in the other organism's suffering. Chimps may never learn peace and cooperation simply because it's not of value to them and they aren't wired to prioritize it. What if we uplift a species to sapience and the values they adopt do not agree with our own (e.g., valuing the right to prey on others as a highest order value, compare the Bitenic Squids of Orion's Arm)?
Indeed, some have even argued that increasing social behavior disproportionately increases selection for sociopathic behavior, because individuals that live in a large group but can trick the rest of the group into covering the costs of existence and mooch off of them have a disproportionate advantage. Obviously not everyone can be a social parasite, but it is much easier to be one in a large group than if you live alone.
This kind of goes back to "but why though". The different temperaments of the various great apes go back to what work the best with the environments they exist in. Bonobos are thought to be passive because they live in environments with high densities of fruit and little competition. Humans evolved large brains due to both living in harsher, more unpredictable environments than other great apes but also because we adopted a more carnivorous diet to compensate for the lack of fruits in a savannah environment. Notably brain size spikes in early Homo at the point in which we stop being occasional consumers of flesh and more active predators. If selection for larger brains were present in the chimpanzees' environment they would have already developed larger brains, and more to the point they were already being excluded from this niche because humans were filling the ecological role of "adaptable but necessarily more omnivorous large-brained primate" better than the chimps were.
If humanity cannot afford its closest relative the chance to reach the next evolutionary milestone. How dire is their failure to allow Evolution to take its course?
Again, there is no evolutionary milestone. Evolution is not goal oriented, it merely selects what works in the moment. We only see evolution in terms of "big brain=good" because that's what makes our own species unique. A dolphin might see evolutionary "good" in terms of increasing sensory ability with echolocation, or a tree might perceive evolutionary "good" at one's ability to spread seeds.
Isn't there a conflict between conservation of species and conservation of natural evolution?
In general, the reason this doesn't come up as a conflict is a matter of scale. It doesn't matter what a species "evolutionary potential" is if they are threatened with going extinct in the moment. Humans are so good at monopolizing resources and wiping out other species that it is likely that we'd have wiped out every land mammal larger than a coyote if we hadn't intentionally held ourselves back and adopted conservation ethics. For example, at the turn of the 20th century the white-tailed deer and the wild turkey were almost both driven to extinction, which seems mind-boggling given how abundant these animals are in rural and semi-urban spaces today. Similarly, to use your cattle example cattle (and indeed, most domesticated mammals) currently exist only in domesticated or semi-domesticated form, their agriotype (i.e., the wild form), the aurochsen, was hunted to extinction in the 1600s. The logic with conservation is mostly if we don't act to preserve these species now there won't be anything left to evolve in any direction.
Question: Has anyone taken up this line of reason as a critique of human efforts at conservation?
I've seen several researchers say we should just let things like giant pandas go extinct because they are so bad at surviving it is almost a waste of time and effort to try to save them. Basically the amount of resources being spent to save the giant panda could be used to easily save three to ten species that aren't as self-destructive from an evolutionary perspective.
There is also the rather taboo topic among zoo biologists that the idea that we are preserving endangered species in captivity to restore the native populations if they ever go extinct is mostly bunkum. Zoo populations are severely inbred due to lack of founding diversity and have experienced extreme accidental selection to be more docile as well as exhibit smaller brains, less powerful jaw muscles, and possibly even less intelligence. This is mostly because individuals not amenable to captivity typically die or do not reproduce, whereas the most docile individuals that allow themselves to be handled thrive. The problem with this is that once animals are in zoos it's next to impossible to reintroduce them into the wild because they are so used to human contact. Most large animals such as birds or mammals rely on parental care and social bonds, so it's near impossible to get young individuals to adjust to a wild setting because they learn how to survive from their older conspecifics. Cases where zoo populations are used to restore wild species like black-footed ferrets and California condors are more exceptions that prove the rule and very, very hard to do. So basically if these species go extinct from the wild they are gone for good, and the individuals in zoos are more curiosities and useful ambassadors for raising interest in conservation than anything else (as well as ways to perform behavioral/physiological research without negatively impacting wild populations).
Consider a more extreme example like cattle, the species could be considered one of the most successful ever in terms of population and distribution. But their "adaptations" are forced to suit human needs, were we to return a few cows to the wild they wouldn't be likely to survive. Humans have taken away their evolutionary potential.
What is "evolutionary potential"? Again, it kind of seems like a conflation of evolutionary "good" being teleological selection towards more human-like behavior, rather than reproductive success or geographic distribution. The problem with this is it gets into the question of "what is good" with regards to evolution. A good example of this is the nematode Caenorhabditis elegans. This species is well known in laboratory settings and there is a particularly known laboratory mutation that causes an individual to be born without an external opening for the uterus. As a result, the offspring grow and grow inside the mother until the mother bursts open from the inside and dies. However, it's also been found that this mutation also causes the individual to have higher-than-average numbers of offspring at larger sizes (and hence more likely to survive to reproduce in a laboratory setting).
Therefore, in a purely Darwinian context ("good" is life seeking to maximize its number of offspring) this mutation is a net benefit, but from the perspective of most ethical systems this mutation increases the organism's suffering and is thus bad.
Ultimately, though, isn't it kind of arrogant to presume that our existence as humans represents the pinnacle of life, and all organisms should strive to be like us? How is that different from colonial powers seeking to impose their own values on all other civilizations because they believe their own values to be the best (e.g., white man's burden)?
As a counterexample to the case of cattle you mention, consider the Bandersnatchii from Larry Niven's Known Space. The Bandersnatchii were engineered as sapient food animals by the Thrintun, who for plot reasons preferred the taste of sapient flesh. The Bandersnatchii were engineered without prehensile limbs and were twice the size of a sauropod dinosaur but were engineered to be mutation-proof, so they were unable to develop tools, make art, build spaceships to escape their situation or do much more than eat, and because they were mutation-proof they could not evolve to escape their lot in life. Their existence is described as a living hell because they were trapped for millions of years in their own bodies as intelligent cattle who were barely able to exercise their intelligence on the world around them. They are even explicitly compared to physically handicapped humans in this regard. How would that be ethical to select for cattle to evolve greater intelligence when their existence would be hellish due to lack of ability to manipulate the environment or self-determine?