Yes, I'm well aware of that study and it kept me up at night a few times too.

Sorry I don't have a great answer to that. I don't think human population will drop to 0. That's a bit crazy and unlikely (gut feeling).

But it might be a few more generations longer than we think before population will grow again in developed countries.

maybe its just the planet reverting to equilibrium at the end of the day. just a few years back overpopulation was the concern du jour.