Peter Zeihan's take is that urbanization leads to less children because there's less space, you don't need the free labor kids provides on the farm, and children are very expensive in the city. This is coupled with the speed of urbanization for countries that industrialized after the second world war- the later you industrialize, the faster that industrialization happens, the more stark the transition to a childless…
I wonder if some kind of mandatory civil service will become necessary. I feel in the US that many people are disengaged from the rest of their communities for numerous reasons, and it would be both a direct public service and a benefit to societal cohesion for all people to experience being a servant to their cities via public works.