"Is it possible that existence is our exile and nothingness our home?"
-Emil Cioran, in 'On the Heights of Despair'
Aquinas regarded having been Created, as a blessing from God, a gift. So no wonder he could regard existence as having a greater degree of perfection than non-existence, his theological attitudes required it. To hear a proper challenge to such assumptions, you need to go to the Philosophical Pessimists, like Cioran above.
David Benatar makes an explicit argument that non-existence is better, in his book 'Better Never to Have Been: The Harm of Coming into Existence'. His case is that there is a fundamental assymetry between causing a being to exist vs not exist, in that suffering is a harm and it's absence is good, but absence of pleasure is not bad, unless there is somebody for whom this absence is a deprivation.
I see the value of Cioran and Benatar, and indeed Schopenhauer, as giving a workout for people who for whatever reason have not yet experienced reasons to wish never to have been. Like how young firebrand idealists in politics, should have to respond to Hobbes and Locke and so on, regardless of the unlikelihood they will become converts. We should try to expose ourselves to experiences and minds not like our own, to make sure we don't just drift into habits of mind, but choose them.
Personally I suspect good philosophers, like good artists and poets, will not limit themselves to a pre-defined domain of emotion and experience, but instead seek to plunge into examining the world as they find it, and speak to that. That is the path of authenticity, and before arriving at dark places it would be wise to learn of tools from those who have been there.
Camus identifed a fundamental anguish about our need to find meaning in the world, and the world's inability to give it to us. We must make our own meanings, but then we play a shell-game with ourselves, pretending the thing we made had to be that way for whatever reasons. But Camus argued the honest response is to recognise our situation is absurd, we confront a fundamental disconnect between what we want and what we can have, we wish to imbue our made-up meanings with some greater weightiness.
A lot of superficial readers into Buddhist theology, don't recognise it's fundamental pessimism. It's theology identifies the root-causes of our suffering in not grasping the disconnect between what we want and what the world can give, summarised in the Three Marks of Existence: impermanence, unsatisfactoriness, and no unchanging self or essences. These chime very well with our modern science: with entropy, with the psychological recognition we motivate ourselves seeking things we won't be happy if we get, and things like the Bundle-Theory of the self. But for Buddhism, there is also no escape, there is no non-being, our desires give rise to dependent-origination, until we can understand a way to seperate how we use our minds from the wishes for other things in other places at other times, and instead simply reconcile with right now. In Buddhist thought we are each the latest stitch of an infinite thread.
The fundamental objection to the question you pose is, it involves a counterfactual. Could the universe have been the same, except for a being's non-being? It is cognitively useful for us as humans doing what we do, to have fantasies of counterfactuals, like, what if the dice roll had been different. But in games of chance, or quantum experiments, we purposefully pare down the complexities, to make multiple instances comparable. In reality, there is just one timeline, what happens happens, and if you have the blessing or curse of having come to be, then here you are to find how to situate yourself in regard to it. This very moment, the breath you are taking, will never be again. And right now, you have this vast freedom to decide why to be, how to be, a freedom we constantly seek to avoid or diminish. And cannot escape.
If you welcome this very moment, existence is a blessing. For Buddhists there is a way to situate ourselves to welcome all moments, and it is deeply linked to recognising we don't need to wish anything is different, as we express being this instance of the universe. By recognising intersubjectivity, we can see there is no isolated being or not-being, but we arose together, and the proper attitude is then concern for all sentient beings, from your unique moment. Wishing for something else, cannot help you be present to what you are.
