Treating aging is not the same as treating death. The reason the two are colloquially linked is because aging has been by far the dominant driver of death for all of human history. To your point, aging has never failed to kill a human.

There are a couple reasons to believe that aging is malleable.

One is that we know how to de-age human cells using the same factors that fertilized oocytes use to de-age the underlying egg cell (typically many decades in age). https://en.wikipedia.org/wiki/Induced_pluripotent_stem_cell

The second is that nature has examples of biological immortality. Planaria are a fascinating one as they have among the trashiest, most mutated genome on the planet. They're even polypoid, where they have multiple sets of chromosomes, but they're simultaneously the best regenerators on the planet. Despite having a discrete nervous system and centralized brain, you can slice a planaria into hundreds of pieces, and each of those will regenerate into a full, functional adult. They don't age, they don't get cancer. The issue is that we have no idea how this happens. So there is still a lot for us to learn about the fundamentals of aging and regeneration overall.