This overlooks that a major reason why future-us won't care is because the issue will be A) settled or B) made irrelevant.
People have less ardent opinions about, I dunno, persistence hunting or blacksmithing or the-concept-of-property-rights-at-all because either one side conclusively won the argument or because it ceased to be a need or bottleneck that people felt it was important to struggle over.
I think it's a good reminder to check whether the thing really doesn't matter, in the moment, but if the reason for not caring in the future is A, this is actually more reason to care now, when there's still a chance to influence things, and if the reason for not caring in the future is B, that's ... not really relevant? If our great-grandchildren won't care about pollution because they have infinite clean energy that doesn't really make it not-matter today.
Some user named Dan L commented on an ACX post:
[...]things can feel more intense now than ever before because more people now care about {things we care about now} than ever before. People used to care less about {things we care about now}, so past times feel like they were less intense. But actually, people used to care *a whole lot* about {things people used to care about}, which we don't care nearly as much about now. So people back then would have felt like things were uniquely intense, and would look at modern disagreements about {things we care about now} and not really get it.
This is all very tautological and makes the very dry statistical bias obvious, but requires confronting that things we used to care about aren't the things we care about now, and almost certainly won't be the things we care about in the future. It is not easy to accept that the answer to "who will win this culture war?" will very possibly be "you won't care". Oh, you'll still have an opinion all right, but *this* fire in your belly will fade faster than you thought possible. As it will for nearly everyone else. This too shall pass.
I found this very powerful when I came across it, and I have been trying to keep it in mind ever since.
(See also: https://en.wikipedia.org/wiki/This_too_shall_pass )