It’s not obvious. Low birth rates are completely sustainable, you just kill anyone who can’t afford to retire and can’t work anymore, and society functions perfectly well.
While the alternative is everyone who is unable to wotk is killed anyway by the apathy of the system?
We are doing what you are describing already, in the system we currently live in.
What? In the current system we pay retired people money based on past employment as well as just for living long enough, in most countries. Japan can no longer do that soon because without taxing their young to poverty, they just don’t have enough income to fund it.
If you think unemployment or SS pay a living wage you have things to go learn
We have machines that can do the work of 100 people in the past
I’m sure that we could make it work without killing anyone