Spent a few years doing population projections for regional planning and the thing that always got lost was how sensitive the long run numbers are to small fertility and migration assumptions.

Relevant xkcd - https://xkcd.com/605/

I think we're already at a doomsday for population. The inertia of such a large world running on capitalism means we can't do anything about our problems any more, so I feel the bullet has been effectively fired. We're just watching it in slow motion.

The real "doomsday" is the fact that we no longer have the population growth to sustain the increases that markets demand while simultaneously not alienating the ever-loving hell out of the labor and customers that capital depend upon for those increases. It's already leading to socio-economic and political problems.

I’m as skeptical of underpopulation doomerism as I was about the previous overpopulation panic.

Both commit the fallacy of assuming present trends remain in place indefinitely and ignoring technology.

They also both tend to be racist. Overpopulation was always about too many of the “wrong” people while underpopulation is about not enough of the “right” people.

"The great vice of the Greeks was not sodomy, but extrapolation"

I've got whiplash from the speed at which we've jumped from overpopulation doomsday to demographic degrowth doomsday.

It seems likely we lend too much credence to experts ability to model complex systems over a long period of time.