> All it takes is one subgroup to keep reproducing above the replacement rate, and after enough generations they will come to dominate and overall growth will continue.
Consequently, religion is going to thrive. Everyone else stops reproducing once they reach a certain standard of living.
Perhaps that's the point. Religion is universal across human cultures, it must serve some, as yet unknown, evolutionary purpose to be so universally widespread. I doubt that historical purpose was higher birth rates, but it certainly is now. Since it seems to grant such a huge advantage to evolutionary fitness, it will thrive, even dominate.
Consequently, religion is going to thrive. Everyone else stops reproducing once they reach a certain standard of living.