> nature has already optimized for maximum human lifespan
Doubtful. The bowhead whale, which lives over 200 years[1], has mutations that we lack. Given that mutations happen randomly and that the dolphin genome is basically the same as the ours (with a few chromosomal rearrangements)[2], it suggests that bowheads got lucky in ways humans did not. A mere fluke.
Evolution hasn't finished with us, either.
It'll be great when we finally break through it. Think of all those extra years we can spend looking at websites on our cell phones.