Humans live for decades. Catastrophic climate change is less than a generation away. It would take hundreds of years for the population to shrink enough to stop it.

That's not how "natural self-regulation" works. People aren't PID regulators and don't suddenly adjust their fertility to not exceed the carrying capacity of the environment.

Natural self-regulation means hitting resource limits, followed by sudden and drastic mass dying off, through wars and starvation.

Yes, a decreasing tax base is terrible for societies.