My 2*(grand)mother raised 11 children. From what I've heard from other family members, the key to success is not giving enough emotional and material support to each child.
What I'm trying to say is: the expectations on children quality of life are also much higher vs 100 yrs ago.
It's not that we can't afford more children, but that we can't afford to give more children and ourselves the level of comfort we've come to expect.
I could afford to give quite a few children the standard of living we had when I was a kid, but while it was fine for it's time, it's not what I'd want to go back to for either myself or my kids.
It's also why I don't think this is a crisis in the long term on a global basis, though it may become one for individual countries:
Once wealth catches up to a level where we have sufficient buffer, I think we can balance things out.
https://www.nytimes.com/2023/06/09/opinion/ezra-klein-kriste...?
"We could be raising our children cooperatively in dwellings designed to enhance rather than inhibit social connection. We could be more sustainably sharing our resources and teaching our children to value collaboration over competition. We could be living in wider lateral networks of love, care, and support; fighting the scourges of loneliness, anxiety, precarity and the many stresses associated with modern life. Most of us are not.”