People in developed societies tend to want more than the minimum. They want to invest time in something that makes them fulfilled and not merely spend their entire life raising babies. Further they recognize that given finite resources that they can do better for fewer kids.
This isn't much of a problem in current developed nations, I don't see a good reason to suppose it will become so.
Family focused people realize that finite resources available to them are best invested in fewer kids. How many of the people that find raising babies the fulfilling part are raising 8?
This isn't much of a problem in current developed nations, I don't see a good reason to suppose it will become so.