[sdenton4]

"Such women will be replaced by women who, genetically or memetically, want more children."

That's not necessarily true. The rural population has shrunk significantly, even as rural family sizes tend to be bigger. The kids largely move to cities (following the gigantic trend towards urbanisation, and in responses to economic realities), and then proceed to have fewer kids in the urban context.

If there's a context with high birthrates, you've got to account for replacement rate within the context: there's a tendency for the kids to find their way to other contexts and thus revert to the mean. Insane "quiverfull" people have lots of kids, but it's not clear to me that they manage to keep up the crazy across generations.