It relies on the birth rate in other parts of the world to stay high (something you can never guarantee), and requires that those countries stay poor enough that they want to leave everything behind and emigrate (something you can't guarantee, and also something shitty to try to guarantee).
Not really, most countries are dealing with it by increasing immigration. That's clearly not a sustainable long-term plan.
Why not? If you manage immigration such that the population stays constant, what's bad?
It relies on the birth rate in other parts of the world to stay high (something you can never guarantee), and requires that those countries stay poor enough that they want to leave everything behind and emigrate (something you can't guarantee, and also something shitty to try to guarantee).