The most important work of "How to Build the Future" is political work - reforming our property relations, for example, so that we aren't organizing our economic lives around feudal holdovers like land titles. Who gives a shit if Elon Musk can connect his brain to the Internet and live forever as a sentient AI, while the rest of humanity drinks ditch water and lives small, dull lives?
And with that in place, thousands of other smaller problems evaporate along with it.
That's not going to be a fun world to live in, especially if the treatments requires biological raw materials that the destitute can sell.
What makes you believe that life extension will be any different? Or, if you disagree that expensive treatments generally stay expensive, what are your examples?
I agree we could have a temporary awkward period in the middle, say 20 years, where it's not cheap yet. But on the scale of history that's a short period of time...I'll admit that's cold comfort to those who die in the meantime.
(See: antibiotics, insulin, appendectomies, lasik, ...)
My counter-counter example would be dentistry, or various forms of surgery in general. Especially the latter is expensive as hell, but most of the world managed to create systems that give access to it to pretty much everyone. Even the US somewhat manages that.