Whether something is doable is irrelevant when it comes to determining whether it is right.
A separate question is what should we do, which is different from what is right. We should definitely do the most right thing we possibly can, but just because we can't do something does not mean that it is any less right.
A real example: There's nothing we can realistically do to stop much of the suffering undergone by wild animals through the predatory instinct. Yet the suffering of prey is very real and has ethical implications. Here we see something which has moral standing even though there appears to be nothing we can do to help the situation (beyond some trivial amount).
One of the criteria moral philosophers use to asses the credibility and power of a moral theory is "applicability". That is, how easy is it for humans to implement a moral rule? For example, a rule exists like "donate 23 hours a day to charity" it would be impossible for humans to fulfill the goal.
This lead me to start thinking about whether we want to be able to to pursue "the moral theoretical truth" should such a truth exist, or if we want to find the most applicable and practical set of rules, such that reasonably intramentaly rational (human) agents could figure out what is best in any given situation.
I feel like this is sort of like a map-territory distinction in a loose way. For example, the best thing to do in situation X might be A. A may be so difficult or require so much sacrifice, that B might be preferable, even if the overall outcome is not as good. This reminds me of how Eliezer says that the map is not the territory, but you can't fold the territory and put it in your pocket.
I'd love to be able to understand this issue a little better. If anyone has any thoughts, ideas or evidence, I'd appreciate hearing them.
Thanks,
Jeremy