alt Hacker NewsThe concern is that if the birthrate drops low enough that having kids becomes unusual, it causes societal changes that create a negative feedback loop that will continue pushing the birthrate lower and lower. If the number of elderly far exceeds the number of working people (which is already locked in for South Korea), you then have to figure out how to restructure society around this while maintaining social order.
You can see this in a microcosm at churches - the ones full of families and exploding children will continue to have such; and the ones that are newly wed or nearly dead will chase families with children away even if entirely unintentionally.
If it happens on a societal level it's tremendously powerful.