Encouraging high birth rates or even replacement birth rates in relatively free societies with decent standards of living and equality for women is something that has not been achieved yet. Japan hasn’t solved it, and neither have we - if not for legal and illegal immigration, the US population would be shrinking right now.
“what did we know in the 40s and 50s that we forgot?“
The Gospel.
And biblical philosophy in general.
Evangelicals are procreating beyond replacement. We got this.