Population is an extremely complex, dynamic system, and I don't think we have any way of actually predicting it -- all we can do is look at trend lines and make projections.
(caveat - not a social scientist; just my current opinion; etc.)
Population Bomb's core claim was about the instantaneous rate of reproduction. This is a complex stochastic process. It could drop to 0 overnight if people decide no more babies.
But population decline is easier to model mid term because you don't need to make almost any assumptions. The next 18 years of university intake are all already born and there ain't a lot of them. The only way for them is down.
Clearly, what's beyond that is hard to forecast, but even then making a pretty good forecast for the next 25 years only depends on forecasting births in the next 7 years.
Maybe we'll have a billion humans living in orbit in a century. Unsure if they'll be willing to pay Earth Tax though.
At small scales you can have them give up their culture and assimilate but at replacement scales you just turn your country into a third world country. That's completely unacceptable.
One is invited, the other is unwanted.
No one wanted or invited any of that.