TL;DR: life expectancy is longer now than ever before, so more women in the reproductive bracket, and this basically cancels out a falling fertility rate.
The interesting point here is the concept of “population momentum” which acts like a capacitor that buffers decreasing fertility rates in the short term - yet - “peak child” has been reached.
I think the takeaway here is people who extrapolate global population in 2100 at 15+ Billion have it wrong.
You're looking at the wrong thing. It's the life expectancy of the children that matters most, not the life expectancy of adults into old age. The reason the number of women in the reproductive bracket has risen is that more of those births are surviving to child bearing age.
We'll see soon if this theory is correct with globally stagnating population or not.
A non-increasing population leads to economic stagnation (at best). That, coupled with a decline in religiosity leading to a loss of societal cohesion will eventually bring down the whole house of cards that is modern civilization.
Famine, war, etc. follows. And we're back to a less populated world with high infant mortality again. Hard times lead to a return to religiosity and large families.
Hopefully this all happens after I'm gone.
You don't get that in secular society.
You can't just take a core element of a heavy intertwined, complex system without collateral damage. It's like deleting a file in a spaghetti codebase. Half the codebase interacts with any given file in some way or other.
Kind of like in agriculture, there's "intensive" economic growth and "extensive" economic growth. Intensive being when the same number of people are more productive and extensive being when you have more people.
You can have economic growth with a stable population. Heck, Romania's having economic growth while its population is declining! (a population loss of about 10% in the last 15 years, during which time its economy doubled)
I also don't agree the consequences have to be 'very' negative. However that's such a vague and undefined term, it could mean anything. I believe the costs are manageable and that a stable, sustainable economy can be compatible with stable or rising living standards.