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.