> 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.