I don't understand how your pushback is rational. It is literally true that replacement is 2.1 children per woman on average. It is literally true that a population that doesn't meet this threshold is literally dying out. It is literally true that no wealthy population has children at replacement rate or higher; all wealthy populations are dying out. None of these facts are disputable.

Do you deny that that subset of people in wealthy societies who are atheists are also having children at lower than replacement rate, just like the general population?