I doubt that the rising difficulty of energy extraction has anything to do with population leveling and decline. For advanced economies, it's the social changes

I doubt that the rising difficulty of energy extraction has anything to do with population leveling and decline. For advanced economies, it's the social changes that allow fewer children or disincentivize them. These were likely enabled by cheaper energy in the first place, but if the energy remained easy to extract they'd still be in motion. Rising housing prices is a very direct result of pumping out more people and it has maybe a little to so with fertility rates. Who wouldn't want the cost of rent to go down? Rent-extractors, primarily. Many fewer of them then renters, so this seems like an economic good. That said, the comfortable carrying capacity of the Earth for human life (and all the other things still surviving with us) seems to be far less, so building systems to operate well under contraction, and working to contract to some rough level and the level off with a new system, seems prudent for the long haul.