And that's fine, but now you don't have the right behavior in the limit to defeat longtermism, because with the right assumptions population growth can easily s

And that's fine, but now you don't have the right behavior in the limit to defeat longtermism, because with the right assumptions population growth can easily swamp the effects of a delay. Unless I'm missing something you really do need the value of a life to approach zero in the distant future for your argument to work. I actually agree with you that longtermism is problematic; I'm not fond of ethical systems that give wildly shifting directives based on guesses about the shape of the future. But that's why I'm not utilitarian. Grant all the assumptions and the logic seems pretty sound to me.