I bite this bullet to an extent, but I don't think the argument that strong. If someone has a better-than-average life before they die, they can still raise the average, especially if everyone else dies too. I'm not sure how to model that easily; I'm thinking of something like: the utility of a world is the integral of all the utilities of everyone in it (all the utility anyone ever experiences), divided by the number of people who ever existed. In this framework, I think it would be permissible to create a mortal person in some circumstances, but they might be too rare to be plausible.