Even within pleasure- or QALY-utilitarianism, which seems technically wrong, you can avoid this by recognizing that those possible people probably exist regardless in some timeline or other. I think. We don't understand this very well. But it looks like you want lots of people to follow the rule of making their timelines good places to live (for those who've already entered the timeline). Which does appear to save utilitarianism's use as a rule of thumb.