This is quite farfetched. This takes kin selection--already difficult to get to work--and pushes it into a timescale of existential environmental shocks.

The impact of having a majority of low-diversity old individuals would tend to be felt by having non-catastrophic environmental stresses kill off some of the individuals (especially those in more marginal environment), leaving the younger and more diverse crowd to selectively repopulate (if, indeed, they had an evolutionary advantage).

You seem to have fallen victim to your own point about teleology here. Evolution has no means by which to recognize that something will work better in a different context. It's all local, all the time. This is why it's tricky to get kin selection to work: you have to help so many kin so much for it to outweigh any effect on you yourself that it's difficult to come up with scenarios where that can happen. (Reciprocal altruism, in contrast, is much easier to set up.)