[A_D_E_P_T]

Fertility is not a heritable trait; it is a norms-based, cultural trait. This was recently highlighted in a good paper by Greg Clark, author of A Farewell to Alms: https://www.sciencedirect.com/science/article/pii/S1570677X2...

Once fertility declines, it's extremely hard to reverse the decline without a massive cultural shift. Countries like S.Korea and Hungary have made heroic efforts -- without much success. People raised in a generational culture of small families tend to have small families themselves. So either your culture shifts, or your culture is quickly replaced by high-fertility subcultures such as the Amish, Quiverfull, and Haredim. ("Quickly" in generational terms, of course.)

My thinking is that if you value your culture and its values, you should want to see it persist. The only way for it to persist in meaningful terms is for it to reproduce to at least a replacement level. The alternative -- which might be better for the environment in the short term, but not on very long timescales -- represents not a temporary lull but probably a permanent cultural decline.