Biological altruism is not unique to humans; other animals, whose adaptations for altruism, are presumably much worse at conscious considerations of this kind. In addition, the common nature of one-shot PD-esque encounters, where you'll never see the other player again, is in many ways a unique byproduct of the modern world, meaning that there would be little selection pressure for an adaption that defects on the last round.

Note that I did discuss an argument like this (such that the variant which defects on the last round is more complicated, and the last round or single-round case happens rarely enough that it can't be selected for). But it strikes me as implausible, particularly for human beings. There's something "off" with the theory of reciprocal altruism if it relies on that defence against last round defection : the prediction would be that the cleverer the species, then the more likely they are to detect last-round cases, and the more likely reciprocal altruism is to collapse over time. So we would see less reciprocal altruism in cleverer species, but the opposite is true.

Incidentally, we could try to test the theory in vampire bats (also known to practice reciprocal altruism). Do bats stop reciprocating after a certain age (because death now too close)? Is there any selection pressure for that age to slowly creep forwards in a given population to younger and younger ages? If so, is there a counter-pressure which stops the creep forward, and what is it?

Evolutionarily speaking, what matters is the genes; even after a person dies, their foes can retaliate against their kids.

Yes, that's theoretically sound i.e. this could stop last-round defection ever entering the population. Again it could be tested in vampire bats and other reciprocating species.

It's kind of a grizzly thought though, isn't it? That reciprocal altruism is maintained by vengeance against a whole family... It suggests that cultures which taboo such vengeance will eventually collapse (from last round defection creeping forwards), but there's no evidence of that as far as I can see. Perhaps the taboo is too recent, and the effect hasn't set in yet.