"Luckily". Why do people keep seeing this as a positive? Isn't it a disaster? What's the end result of below-replacement birth rates? Extinction? Some new equilibrium where we don't have enough civilization to do social welfare and have to start depending on our own children for old-age care again?
Just like population growth will not continue indefinitely, neither will population decline.
Below-replacement levels signal that there are a lot of individuals in the pool who are not pre-disposed to reproduce in their current environment.
Over time, this will be selected against, those individuals who are pre-disposed to reproduce will produce more individuals similar to them.
That pre-disposition is genetic to some degree, but it is also cultural. Our society tells women that it's okay to defer birth or to not have children at all. Other societies pressure women to be married with children by the age of 25, before their fertility declines.
From a moral perspective, the former may be preferable, but from a population maintenance perspective, obviously the latter is advantageous.
As for social welfare, obviously there's a looming crisis of elderly care, but it can be mitigated to some degree by immigration.
There's no reason to assume lower birth rates would continue all the way to extinction. Even if they don't rise again naturally, they could be raised through explicit policy. In either case, we're a long way from having to worry about extinction due to low birth rates.
Conservative religious minorities (such as Amish and ultra-Orthodox Jews) manage to maintain big family sizes and retain 80%+ of their kids in the parents’ religion.
They are small minorities now, but give it a couple of centuries and they may grow to become the majority of the population.
A future dominated by religious conservativism is not the future for which many hope or expect
