Religiosity isn’t declining (globally) and the book addresses the “defection” idea, noting that most people still have similar beliefs as their parents, and that birth rates still outcompete defections.
Even if "most" people don't "defect", you have to make up the remainder by people going the other way, for which there doesn't seem to be any evidence.
https://www.weforum.org/agenda/2015/10/how-religious-will-th...
You don’t need to make up the remainder by going the other way, as I mentioned above: religious population growth comes from birth rates, not conversions.