> At 25 pretty much all your cells have died and been replaced many times, and if over evolutionary times we had lived to be 500 years on average, we'd have mechanisms to keep up healthy that long.
You're missing the point that some cells, essential to our survival, are never replaced, and when they decay, we die. One example is brain cells -- they're never replaced, and when they're done, so are we. This means that, unless we can get around apoptosis (programmed cell death), then we won't ever move beyond a certain age. We aren't close to understanding apoptosis.
> What is "good enough" for our genes isn't always good for us as people.
Ah, someone who thinks he can outwit nature. When we try to outwit nature, we end up outwitting ourselves.
> Most of what can be called "modern progress" is about protecting us from what would happen to us if nature was left to itself.
You mean, like antibiotics? The antibiotics that we have foolishly mismanaged to the extent that they simply don't work any more?
Contrary to your thesis, we need to figure out how to get along with nature, not pretend we can dominate nature. We've tried to run the show, and nature has responded by showing us how naive we are.
And this idea isn't some airy-fairy New Age philosophy, it's the result of careful scientific work. We're just getting started in figuring out how to get along with nature, and everything we've tried to date has backfired:
Medical and nutritional advances -> overpopulation
Antibiotics and other medical breakthroughs -> the gradual evolution of antibiotic-immune microbes.
Longevity extension -> overcrowding, a plague of chronic diseases in the elderly, deep philosophical questions about meaningless long lives.
> Curing the diseases of aging is just the continuation of that
Curing the "diseases of aging" is not your topic. Your topic is the disease of age. That's not the same thing at all.