When standards of living rise, birthrates drop.
Western countries, japan & korea all have low birthrates and all developed them as av. income rose. China too but one-child made it a less clear example. Even within a society, middle and upper classes have smaller families than the working class.
Declining birthrates can't be fixed by improving living standards, but that's fine because low birthrate is a good thing - a sign of a society doing well.
If a high income/low birthrate society needs more citizens, immigration from low income/high birthrate areas is the only viable option. Endless expansion is a fools errand anyway.