I've been reading and thinking about this topic a lot lately, and this is one of the most well-written and informative articles I've read so far! I do think that some rearrangement of populations seems like the logical answer (rather than individual countries pushing their people to have more children), though as you say, visa rules make that somewhat impractical, in addition to cultural and societal considerations. More and more countries are starting digital nomad visa programs though, and I wonder if that will become necessary in countries like Japan as well.