But what they can do, that AI can't, is try new things in measured, effective, and ethical ways.

And that hypothetical "billions of hours of human life saved" has to be measured against the actual damage being done right now.

Real damage to economy, environment, politics, social cohesion, and people's lives now

vs

Maybe, someday, we improve the speed of finding cures for diseases? In an unknown way, at an unknown time, for an unknown cost, and by an unknown amount.

Who knows, maybe they'll give everyone a pony while they're at it! It seems just as likely as what you're proposing.