> the point is that our current social structures seem incompatible with even replacement levels of child birth.

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.

Um I meant the transition effects are also a problem, not that they are a bigger problem - my wording was a bit slack there. Yes probably we could overcome those issues, but with no clear reason for birth rates to increase as population falls (and quite a few reasons for the reverse - e.g. greater proportion of elderly requiring care that would otherwise be given to infants), our population and civilization would simply evaporate.

Our current social structure was built on wrong assumptions, this is what need to be fixed.

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.

Yes, exponential (has been doubling every ~40 years for a long time, and a scenario of halving at the same rate) is what I was talking about above.