-> Reunite with their families and become retired themselves in the countries that they originally came to to bolster the economy. Which would be what should be expected to happen ->
-> If the social and economic conditions of a country makes is so that the fertility rate is below replacement level, then it will also happen to anyone new coming in which just ->
-> extends the problem into the future, where there at some point needs to be a stop in the growth of the population or even decline long term, things will change fast with automation.
Sidenote: all or nothing immigration is not sensible, the question will always be: how much immigration of which kind of immigration. It is of course a balance thing.
There can not be any nation without it having some sort of borders and criteria for who gets citizenship, nations and the rule of law is there to regulate people within nation states.
It may be that the sort of society that has great economic conditions are the same sort of country that fosters under-replacement level babies, no matter the background of people in it
Exactly, the question is: Do they make enough babies here? If they do not, then the problem is just extended. Can a liberated and industrial nation sustain a high birthrate? ->
-> or is there something about the social and cultural norms that makes industrialized / market economy nations in our current economic system non-compatible with high birth rates ->
-> that may actually be the case, and if so it means that it is not a stable solution long term, that wealth/productivity/work culture at some point becomes a detriment for a nation.
Lower infant mortality alone does not explain the discrepancy of the birthrate, and if economic incentives for having babies work then it is an alternative to replacement immigration.