I grant all of your points. But I think there would be some very clear upsides as well, assuming that longevity is available to more than just a few billionaire

I grant all of your points. But I think there would be some very clear upsides as well, assuming that longevity is available to more than just a few billionaires. Years spent observing a field or topic with a well-trained eye is incredibly valuable. But by the time you're an expert, you typically have at best 3 decades and often much less to practice before serious decline kicks in. It would be a very different world where the number of experts and veterans rose by an order of magnitude. Also, as I've come to see with my own advancing years, humans are very good at forgetting history and its lessons. The best way to maintain historical awareness is to simply have people around who lived through it. The current historical moment is an illustration of this - the world is sliding toward large-scale war just as the last of those who saw ww2 with their own eyes are leaving the scene. I don't think that's a coincidence.