I don't really think immigration is a long-term solution, because of the scale issue - which most greatly underestimate. We're talking about needing a never-ending stream of hundreds of millions of people. And you'd ideally want people that speak the language, have at least some basic skills, and so on. It's not particularly realistic, even before getting into the social chaos that such would cause.

And it becomes even less realistic if you look outward to times when this becomes necessary. Japan is a good example of this issue. Migrating to Japan is not difficult. The only meaningful barrier is learning basic Japanese. Beyond that, after just 5 years of residency you can even apply for citizenship which has a very high acceptance rate. And there are a ton of 'Japanese enthusiasts', many of whom already speak basic Japanese.

And many of them have tried to migrate, but they don't last at all. They quickly realize that a Japan in decline is not the Japan in their minds. Getting paid $1500 a month to work a job with extremely high expectations and demands in a country with a median age of 50 (and increasing) isn't the Japan they thought they were moving to.