> Researching immortality or unnatural longevity isn't worth the investment for a society because a society of immortals becomes either stagnant or unsustainable.

This is said with an awful lot of certainty for something that's never been accomplished before. Do people and systems get stuck in their ways? Absolutely. But they also change for better and for worse. There's no guarantee/proof that living forever ends in stagnation or an unsustainable culture.

>a society of immortals becomes either stagnant or unsustainable

Which society of immortals are you referring to?

Tolkein's Elves :)?

More seriously, it's a common theory that age-related diseases are an evolved adaptation because over time a species must form an equilibrium with its resource base.

Yes, but pretty much every other equilibrium based limit is something humans decided we didn't want anymore either.

We are better than other species at filling new niches. But being champion is no excuse for getting cocky.

Our general mode of operation seems to be the very opposite. Getting cocky, causing some very untoward consequences, then learning from them.

I don't even think it is necessarily bad in toto. The other pole could look like stagnation fueled by indecisiveness and fear of possible consequences. I definitely know some individuals who spend their lives like that, and they don't seem to be happy.

The problem with this conversation is that it's about immortality, which no one is going for right now. The best research is looking at addressing specific age related issues in order to increase your healthy years and, potentially, increase your lifespan by maybe a decade.

But the conversation is always "immortal billionaires".

It's a totally fake problem to talk about that.

An immortal fascist dictator may be expected to get assassinated early on, versus people waiting for him to die from natural causes.

The universe is huge and the Solar System alone could probably accomodate 50 billion humans. With something like artificial hibernation (we are discussing a pretty far future here), you could travel to nearby stars as well.

As for stagnation, there is an unspoken presumption built into your thought, namely that older rejuvenated people will gain younger bodies back, but that they will stay mentally old. I don't think this will be necessarily true. It rejuvenation works, it will have to work on the brain too, and rejuvenated people will revert to younger patterns of thinking as well. Only with more memories and experiences, which could actually improve their results. For example, they will be unlikely to join some populist cause if they burnt themselves twice already.