That is a problem for average-over-agents utilitarianism, but not a fatal one; the per-agent utility function you use need not reflect all of that agent’s preferences, it can reflect something narrower like “that agent’s preferences excluding preferences that refer to other agents and which those agents would choose to veto”. (Of course, that’s a terrible hack, which must be added to the hacks to deal with varying population sizes, divergence, and so on, and the resulting theory ends up being extremely inelegant.)
That is a problem for average-over-agents utilitarianism, but not a fatal one; the per-agent utility function you use need not reflect all of that agent’s preferences, it can reflect something narrower like “that agent’s preferences excluding preferences that refer to other agents and which those agents would choose to veto”. (Of course, that’s a terrible hack, which must be added to the hacks to deal with varying population sizes, divergence, and so on, and the resulting theory ends up being extremely inelegant.)
True enough, there are always more hacks a utilitarian can throw on to their theory to avoid issues like this.