Even if you don't think life extension technologies are a good thing, it's only a matter of time before almost everyone thinks they are. Whatever part of "humanity" you value more than life will be gone forever.

Doesn't this argument imply that we should self-modify to become monomaniacal fitness-maximizers, devoting every quantum of effort towards the goal of tiling the universe with copies of ourselves? Hey, if you don't, someone else will! Natural selection marches on; it's only a matter of time.