Yes, but those children that move away also move towards Western norms of sub-replacement fertility; while the children that don't, don't. I wouldn't say the b

Yes, but those children that move away also move towards Western norms of sub-replacement fertility; while the children that don't, don't. I wouldn't say the best way to think about this is "these crazies are baby-making machines!" (Strawmanning your argument a little there.) There is some craziness in e.g. Quiverfull protestantism or Ultra-Orthodox Judaism, sure. Nevertheless, if culture X is set to survive into the future on current trends, and culture Y is not, then maybe culture X isn't completely stupid.