-> 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.