An edge case like that isn't really the point. I mean, what percentage of "responses to old comments" fall into that bucket? My guess is that it's a very small percentage. Weighed against the "propagating / encouraging low-value flame wars" possibility, I absolutely consider the lack of such notifications to a Good Thing.
Especially considering somebody who's particularly motivated to find delayed answers to specific questions can always bookmark the particular comment and check it manually, or just occasionally scroll-back through the "threads" page or whatever. If they're not motivated enough to do that, then maybe the discussion wasn't that important to begin with.
Since "low-value flame wars" are already explicitly against the community guidelines, using that as the argument against notifications is like banning knives because they are potential murder weapons. And then having people defend that decision as a "good thing" that we can't cut vegetables.
You can already flag inappropriate content. We don't have to fear it.
Especially considering somebody who's particularly motivated to find delayed answers to specific questions can always bookmark the particular comment and check it manually, or just occasionally scroll-back through the "threads" page or whatever. If they're not motivated enough to do that, then maybe the discussion wasn't that important to begin with.