A similar calculus suggests you shouldn't work on life extension, if your goal is to live longer. I think both arguments are valid and useful to remember, but they overlook some important considerations, particularly in relation to motivation and social affiliation, and particularly when the project entails a real social benefit in addition to the perceived personal benefit.

Independently, I think you may underestimate the value of building shelters (though its surely not a good play on the utilitarian calculus). On the altruistic account, it's better if I survive in worlds where others don't. And in such worlds I also stand to have more numerous descendants, though not higher quality of life. So I don't think you should invoke that particular argument---that survival is less valuable post-apocalypse---against shelters.

(edit: "if not" --> "though not")