By the time that happens, centuries will have passed and the problems caused by population loss will have already been experienced.
Also, I've worked with and known Mormons, and I've never known them to be huge kid-producers, so I'm questioning this assumption. They have more kids than the current white American average AFAICT, but not hugely so. They tend to be higher income and put a lot of energy and resources into their kids; you can't do that and have 10 kids. In general, it's low-education people who have tons of kids. Mormons are not like this; they're generally very well educated.
Also, I've worked with and known Mormons, and I've never known them to be huge kid-producers, so I'm questioning this assumption. They have more kids than the current white American average AFAICT, but not hugely so. They tend to be higher income and put a lot of energy and resources into their kids; you can't do that and have 10 kids. In general, it's low-education people who have tons of kids. Mormons are not like this; they're generally very well educated.