It’s not enough to say that a smaller population would be okay. Leaving aside the economic catastrophe from inverted population pyramids, why do you assume fertility rates suddenly increase in a highly individualistic society once the population shrinks by whatever level you consider acceptable?
In my opinion the problem of low birthrate is because of (in random order):
- career obsession culture (you're a failure if you don't climb up the ladder, which has done a lot of damage especially to women in general)
- prolonged childhood (30s is the new 20s)
- prolonged education (spend a quarter of a century in child care and schools, mostly with people who themselves have no work experience outside those institutions)
- moral relativity and everything traditional is believed nonsense without even trying to understand it
China/Japan/Korea are far less individualistic than the US but have much worse birth rates. Russia also has terrible demographics and doesn't fit into this narrative.
The determinants of birth rate are probably more mundane than that.
Birth control access Education Rapid industrialization Societal expectations around safety and parental involvement Women working Access to entertainment Cost of housing