And that is before we consider the effects of the second order transition (i.e. going from a growing population to a shrinking population). That in itself wrecks things like pension provision.
This means our current social structures need to change. We can’t have exponential growth forever. It’s literally not possible on a planet with finite resources.
> And that is before we consider the effects of the second order transition (i.e. going from a growing population to a shrinking population). That in itself wrecks things like pension provision.
Pension provisioning will suck yes. But it’s not an “existential threat to our civilization, if not our species. As much as climate change.” You are drastically overstating the drawbacks to population decline, as evidenced by your immediate moving of the goalposts when probed.
This is probably not as difficult as it seems. We talk a lot about the proportion of active / inactive, but we often forget that most population under 20 is inactive and also supported by the active population.
https://en.wikipedia.org/wiki/Seneca_effect
Look at how easily supply chains collapsed due to lockdowns, complex systems are hard to sustain and the focus on efficiency above all else has resulted in reduced redundancy. With less people around it will be hard to even maintain things, let alone improve them
Say we're below replacement rate and the first "halving" has occurred. The replacement rate now no longer is 2.1 (as it currently is), it would be perhaps 3-4. Which won't happen, so then it becomes 5. Which won't happen so then it becomes 6. And so on.
Once the serious decline sets in, it cannot be stopped.
Even if models predict the population growth will slow down and may turn to a decline after peaking 50 years from now, it feels quite rash to over-extrapolate and panic now about a potential eventual decline from the 15 billion peak to a genetic population bottleneck (1000-ish). The population would have to halve ~25 times which would take ~1000 years if it declined at a similar speed as it's been growing now.