Retired voters, and voters approaching retirement, will fight denture and nail against that. The likely result is some claw back of benefits for them, but young workers would still carry a much heavier tax burden than the retired class did, preempting their own retirement savings. That causes them to defer or forego marriage and children as well, causing a continuing downward spiral.

Without an attestupa or genuine efforts to improve fertility rates, we're in for very grim times in most countries. There's a kind of virtuous cycle here, though, as a government that does figure out how to mold its age demographics in an effective manner will be a much more attractive destination for young emigrants.