The US is an enormously attractive immigration target and can easily bring in enormous numbers of new workers if it wants to. It's so good at this that it actually has and those people pay taxes but don't get government benefits.
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.
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.
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.