Your claim is that this trend doesn't just stay the same, that it might reverse. But it's so much worse than it merely not reversing... the trend becomes more extreme. As fertility falls, there are far fewer examples of high fertility for children to grow up around and be normalized to. They lower their own fertility in adulthood in response. Fertility decline is set to accelerate, not slow.

Big claim you're making. Populations are not homogeneous.