The most important part about fallacies is not their name, but finding out where there are errors in the argument, the reasoning or the presentation.
Names may serve as mnemonics or you can explain things better if you pick an example that is functionally equivalent but closer to the real world life experience of your interlocutor, though I've seen far too many people use them as an appeal to authority rather than a reference to be a fan of the "list of fallacy" approach. If you know where the error is, you'll come up with examples (as you did) and if you don't know where the problem is citing named fallacies may end up becoming a fallacy of it's own. I've seen far too many people who scream "strawman fallacy" when you sketch out the implications of their claim rather than trying explain where the problem with the argument actually is and where they find themselves misrepresented or whether it's a misrepresentation at all.
The other problem is that all of the classical fallacies are "non-sequitur" that is "the conclusion does not follow from the premises" so chances are a lot of these fallacies will run by different names for essentially the same problem, precisely because of the point that I tried to make earlier that picking and choosing a good example might depend on your preferences (mnemonics) or your interlocutor's preferences.
While the more informal ones depend on the specific argument and context which, in this case you've not provided enough of.
I argue against trying to increase social equality in your country, because there will always be much poorer countries.
For example the "always" in your second sentence basically expresses a form of fatalism and the believe of futility of the cause itself. If that were to be true and then it would indeed mean that it's useless to engage in these things.
Calling this the Nirvana fallacy or perfect is the opposite of good (shamelessly copying from the other answers to at least fulfill the formal requirements of an answer... If you came for that part give them the credit) only really works under the unstated assumptions, that:
- You can quantify good and bad
- That you can draw a baseline of "good enough"
- And that an action that might not pass as perfect might still either a) be an improvement or b) be "good enough".
And while these assumptions usually aren't too far fetched, you usually are able to formulate a goal, identify improvement and define a "good enough" state. Whether such an argument is fallacious still depends on what you aim for.
Like if you want to research medicine to beat cancer and instead the researchers you hire develop a medicine for a different disease, that is certainly good, but it didn't actually moved you closer to your actual goal. So good indeed came in the way of perfect. Or to be more precise my rhetorical me has fallen for the fallacy of using two different versions of "good", good in terms of the original quest is anything that moves me closer to finding a medicine for cancer, "good" in the second sense is anything that has a net positive for society, the two overlap at times but aren't identical.
And in fact this "fallacy" is notorious for being used to silence opposition, by pretending their goals are unrealistic and that whatever is done by those in power was an improvement (hard to challenge without access to alternatives) and is therefore all they could have ever hoped for and they should be more grateful.
However the devil lies in the details as to whether or not the action passed the baseline level of "good enough" or not and if it didn't then it's potentially not even fallacious to say that it's futile to do it if it's results are not good enough. The other fallacy about that is the tacit assumption that not doing that particular thing means you're not doing anything at all and that not doing anything makes things worse.
But for example there apparently was a time in history where medicine was so awful that not doing anything even made things better, so doing something, that is intended to be positive, but isn't, might even get in the way of something good (regardless of the perfect).
I argue against exercising, because you will never be the fastest runner on the planet.
Similar problem. What is your goal here? Be the fastest runner in the world and build a career off running? Well if you actually will never be the fastest in the world you probably should not dedicate long hours of your day to a goal that is certainly not achievable. (Assuming the argument is true and that it's not just your lack of self-esteem speaking). So with respect to that goal and the intensity of exercise that might not be a fallacy. With respect to exercising at all being a bad thing? Well no it's healthy, it improves yourself in many ways and even if you aren't the fastest in the world there are certainly benefits to exercising so it's not pointless in general just pointless towards one particular end.
I argue against eating vegetables, because it will not make you live forever.
I mean let's be real these aren't credible arguments anyway but the other person just searches for excuses not to do something. That being said if you take these arguments serious than they are not in fact fallacious.
If your goal in life is to do A and action B doesn't help towards that goal, then you shouldn't do B.
So at that point you're not arguing against some rational statements but against a believe, a personal worldview and philosophy which is usually bigger than that particular argument.
So there can be a lot of things in there that are and aren't fallacious.