Not to contradict your point, but I think there are other reasons we don't live to be 900. A human with a 900-year lifespan would be able to reproduce many more times than a human with a 90-year lifespan, so if a 900-year lifespan were easy to evolve then we would have done it by now.
I think there is probably some range of possible lifespans given our architecture. It just so happened that longer ones weren't increasing reproductive fitness (probably for several interrelated reasons) so it was never selected for.
