As are we all - once the demographic decline begins, there appears to be no stopping it. Perhaps only Ceaucescu Romania managed to reverse demographic decline? And he did it by banning contraception, never mind abortion, leading to the notorious orphan crisis in '80's and 90's.

We probably can't do this w/o a full spectrum refactor of society - what work is, how to do status competition, how do we do assortive mating - all has to change, or revert to pre-industrial modes

Climate change will result in many third world environments not being viable. Redistributing their offspring to first world environs that can afford mitigation will forestall collapse.

Alas, xenophobic and self-loathing social darwinist tendencies in the first world will probably prevent this rational solution.

Which is why the first to invent an adult daiper changing robot will be a billionaire.

'Rational solution' to what? The problem of perpetuating the Japanese people, or the problem of keeping the Japanese islands populated?