And basically none of those reasons are related to the two proposed (“getting our s%%% together” or actual die-offs, both kind of cartoonishly silly). It’s largely the availability of birth control, urbanization, and maybe also old-age safety nets.
The whole world is on the way to doing this (Africa will, too, as they will develop… and, in a note of good news, probably become as economically powerful in the 22nd century as Asia is in the 21st). It’ll lead to a cratering of the working population unless we basically start to eliminate retirement (not desirable!) and enact a ton of pro-natalist policies now (in addition to as much immigration as possible).
A thing people don’t understand is that the biggest resource limit is people. Population decline won’t mean abundance but the exact opposite as the dependency ratio shoots through the roof and will be incredibly stubborn to come down (as to try to reverse it with pro-natalist policies will actually make it significantly worse for a couple decades), especially as willing migrants dry up (yes, I realize how counter-narrative that is, but it’ll happen… in the latter part of this century, Africa will become sick of being the cheap labor source of aging Europe and Asia).