I know, I know. Less growth, less innovations and all the things the economists say. But the innovation per capita would be the same. And probably purchasing power parity would be better.
IMO urbanisation and modern tech are a big factor here. If you lived off land in a manner requiring heavy manual labour, then children could really help and provided continuity. Lack of old people’s care was a big encouragement too, and really different generations living together helped with childcare.
Actually this is not so much a problem for the amount of population but of excessive centralization around big cities. The earth, even if we only consider the places with reasonable climate, water and food supply and decent enough orography is pretty much empty.
Everyone agrees that as big cities grow the housing prices increase, but I don't see this happening in small towns far from big cities. Every country is different and obviously there are very good reasons to have some regions not very populated (I'm not saying Lapland should have the same population density as the coast of the Mediterraneum) but in plenty of cases places are unnecessarly crowded.
Spain is a good example of this, Madrid in particular is a city far too expensive for the salaries due to the excessive amount of population. The weather or the land aren't particularly good, it's not that it has massive water supply or anything like that, it's just that there have been centuries of centralization efforts by governments.
Right now we still need warm young bodies to stack shelves, care for old people, reap crops and so on.
With ageing populations you have no less old people but far less young uns to do the work. Also less surplus to do novel stuff too.
Oh and Japan is not keen on immigration either.
Young people mostly just play Nintendo and drink high-balls in Tokyo.
[1] https://www.japantimes.co.jp/life/2020/04/04/food/urban-agri...