[big-green-man]

The problem is that such a decline has intense inertia. When you have declining birth rates, average ages go up. Caring for elderly becomes a more significant chunk of the budget of a society. An only child for example has to care for 2 parents, potentially grandparents, themselves, and if they have a kid, half a kid. When you socialize this, people who have kids subsidize people who don't.

You can see how this would spiral. The society that makes it out of such a spiral is unfortunately the society that stops caring for it's elderly. The transition period would obviously be extremely unsavory and socially catastrophic. What winds up happening is people become incentivized to have kids so that they have someone to take care of them, and those who have more kids inherit the society a couple of generations down. That's what reversal of this trend looks like, it's not just about population numbers, median age is an even more consequential factor.