By the same token, there are animals who have the „live for 400 years“ problem solved so there must be sth left on the table yes?
Other animals have the „regenerate this body part“ problem solved, same.
Other animals have the „regenerate this body part“ problem solved, same.
Note my claim wasn't that optimization isn't possible, just that the impressiveness of the optimization is inversely proportional to the amount of performance left on the table.
In terms of that being a promising avenue, I'd observe all the animals with long lifespans I'm aware of seem to achieve that with a quality of life that would in humans be considered very elderly behavior, due to low energy consumption. People generally want eternal youth, rather than eternal elderliness. I don't know of any 200-year-old animal leaping about from tree to tree or anything like that.
Body part regeneration seems a lot more like a solvable problem to me than generalized "fix aging". While we know we're lacking spontaneous body part regeneration, we also know the "programs" for growing parts are in there, since we used them once.
(As I've observed before on HN, "solving aging" would be a lot easier if we just had to produce organs with the same DNA or something. It's a bit body-horror-esque, but if we could just grow clone bodies and transplant things in we'd probably be pretty close. But there's that whole problem where we'd be happy with a new heart, we'd be happy with a new liver, we'd be happy with a new left arm, but we stubbornly want our old brain....)