(Population x Consumption x Efficiency) / Resources
We can either all consume far less, or there can be less of us. I would rather have half as many people living a 2x more efficient and 4x more luxiourous lifestyle in 2100 than twice as many people fighting for water and arable land.
No sane person wants #2. So if there is a natural/cultural shift that is moving toward depopulation that's a huge win because the other options are doing something similar to China's previous one child policy or some other method of forcing or providing incentives to have fewer children.
As long as society is aware of the trend we can plan accordingly. Demographically, we'll be looking more like Japan, but I don't necessarily see the intrinsic harm in that. The planet has a fixed mass and fixed resources. We can't keep playing this game of "infinite growth forever!" in every aspect of the economy. As the saying goes, "that which cannot continue, will not continue."
we've had a bit of practise over the past handful of decades since the industrial revolution of trying to figure out how to have a roughly stable society under conditions of economic and population growth, perhaps a lot of those rules of thumb need to be thrown-out and rethought
If we’d go back to the Stone Age the climate change problem would solve itself within 100 years.
If CO2 weren’t bad for the environment the climate change problem would solve itself within 100 years.
If economic activity didn’t produce negative externalities the climate change problem would solve itself within 100 years.
Heck, if a giant magic wand were waved around strong enough we might get it solved in 4 days!