I thought there was a broad consensus among social scientists that sub-replacement birthrates in the West are linked to the expense of new household formation, especially wrt. real estate prices. Child support programs can help quite a bit at the margin, but not enough to make a dent in that particular issue. It makes no sense to conflate this situation with Nigeria's, they're polar opposites in many ways.
Everywhere except sub-Saharan Africa and parts of the Middle East have sub-replacement birthrates at this point. Including India and China. China has started seeing contraction, India will start seeing contradiction in ~20-30 years since the measures lag.
It is by no means an issue just in the West.
You're right the situation is different with respect to Nigeria, but the birth rates are also falling in all of the remaining countries. Nigeria's is still high but also falling.
That's not an good example. China has even higher real estate prices(due to land grant fees).