[diffxx]

The instantaneous fertility rate dipping below replacement level does not mean humanity dying out. It might mean that this particular civilization with this particular population level is. Infinite growth ad infinitum leads to an inevitable (and likely catastrophic) collapse. No one can say with certainty what the "correct" fertility rate is. That being said, if a society is set up as a pyramid scheme, it must have infinite growth to sustain itself so I won't argue that this way of life is likely dying.

[atomicnumber3]

We've already passed several Malthusian moments because we invent stuff that allows growth to continue. 50% of all nitrogen in human tissue came from a process we invented like 70 years ago. And we're nowhere near fundamental constraints like energy absorption of the sun onto the earth, and sqft of land per human.