Wait -- the value of children recently changed fundamentally but we should expect no more change far in the future?

No, there are expected changes. We should expect the transition of children from capital to consumption goods will continue as more places move away from subsistence farming and develop old-age pension systems. This will predictably shift selective advantage toward folks with active pro-natalist genes or memes.

This does not reconcile at all with current population trends of developed nations. The UN medium projection for 2050 has the entire world at 2.02. Go ahead and assume people still want children as consumption goods, data suggests that not enough of them want this to maintain even zero population growth beyond the current century.

Perhaps the growth of the high-fertility subgroups (like Mormons, Muslims, Quiverfulls, Amish, Cossacks, ultra-orthodox jews, etc.) will not outmatch the declining fertility of other groups by the end of the 21st century, but they will eventually. Their growth is rapid, exponential, and unbounded. The decline of the sub-replacement groups is slow and bounded (they can't decline to less than zero).