I don’t want to nitpick but the declining birthrate is almost certainly driven by us becoming smarter, not dumber. In most countries the birthrate was propped up heavily by teen pregnancy, which the internet and access to healthcare is slowly eradicating worldwide.
If we as humans can’t maintain our current population without getting high schoolers pregnant, then so be it.
I didn't say declining birth rate is a result of people being dumber.
And I also don't think higher education necessarily means smarter. I'm quite confident that in the next decade, worldwide educational attainment will keep rising with only some temporary setbacks. If making people smart is as simple as sending them to colleges we really have nothing to worry about.
But anyway neither was my point.