There is no "other countries", it's a global economy. Mexico exports $450B worth of stuff to the US every year. When their fertility rate was 6 and then one or two of those kids immigrate to the US, that's fine for them. Now that their fertility rate is below the population replacement rate too, if their kids emigrate their country is screwed. Then there's nobody to make that $450B worth of stuff, because the kids who migrated are busy filling the existing jobs in the US.
Meanwhile what do you expect to happen in countries with fertility rates below population replacement and net out-migration of the youth? Is it morally reasonable to willingly cause that to happen, even without considering the consequences to the US of that level of desperation spreading through the rest of the world?
The alternative would be to get the fertility rate back to the population replacement rate.
Assuming current trends are unchanged we’re still talking about having billions of humans for hundreds of years. On that kind of timescale we might see significant life extension, artificial wombs, and hard core genetic engineering.
Some countries like South Korea are going to face major challenges far sooner, but frankly having the most extreme examples collapse means the average stays higher.
> Assuming current trends are unchanged we’re still talking about having billions of humans for hundreds of years. On that kind of timescale we might see significant life extension, artificial wombs, and hard core genetic engineering.
The absolute number of humans isn't the issue. It's that people expect to retire at 65, but are now living into their 80s and 90s. Retirees have to be supported by working people, i.e. younger people. If the ratio of younger people to older people gets out of kilter, there's huge problems. Life extension makes this worse rather than better.
The ratio of younger vs older people is also a function of biological aging which might look very different in 500 years. I don’t think we can reasonably expect to retire at 65 if healthy lifespan hits 200+.
If 150 year olds are as healthy as current 50 year olds they may very well be expected to work. And personally I’d happily extend how long people are expected to work in exchange for significantly longer lifespans.