Hmm. Yes, it does appear that an less-happy-than-average person presented with a device that would remove them from existence without externalities would be compelled to use it if they are an average utilitarian with a utility function defined in terms of subjective quality of life, regardless of the value of their experiential utility.
The problem is diminished, though not eliminated, if we use a utility function defined in terms of expected preference satisfaction (people generally prefer to continue existing), and I'm really more of a preference than a pleasure/pain utilitarian, but you can overcome that by making the gap between your and the average preference satisfaction large enough that it overcomes your preference for existing in the future. Unlikely, perhaps, but there's nothing in the definition of the scenario that appears to forbid it.
That's the trouble, though; for any given utility function except one dominated by an existence term, it seems possible to construct a scenario where nonexistence is preferable to existence: Utility Monsters for pleasure/pain utilitarians, et cetera. A world populated by average-type preference utilitarians with a dominant preference for existing in the future does seem immune to this problem, but I probably just haven't thought of a sufficiently weird dilemma yet. The only saving grace is that most of the possibilities are pretty far-fetched. Have you actually found a knockdown argument, or just an area where our ethical intuitions go out of scope and stop returning good values?
I don't think that existence is always preferable to nonexistence. A good life seems to have value, but a bad life is, ceteris paribus, not preferable to nonexistence. The problem with average utilitarianism is that a good life is declared worse than nonexistence if it is below average, but I want to preserve the value in that life, not eliminate it. In general, the solution is not to add a large existence term, but to make sure that the utility function as stated matches your actual preference for whether or not someone should exist.
...Have you actually fo
Some people see never-existed people as moral agents, and claim that we can talk about their preferences. Generally this means their personal preference in existing versus non-existing. Formulations such "it is better for someone to have existed than not" reflect this way of thinking.
But if the preferences of never-existed are relevant, then their non-personal perferences are also relevant. Do they perfer a blue world or a pink one? Would they want us to change our political systems? Would they want us to not bring into existence some never-existent people they don't like?
It seems that those who are advocating bringing never-existent people into being in order to satisfy those people's preferences should be focusing their attention on their non-personal preferences instead. After all, we can only bring into being so many trillions of trillions of trillions; but there is no theoretical limit to the number of never-existent people whose non-personal preferences we can satisfy. Just get some reasonable measure across the preferences of never-existent people, and see if there's anything that sticks out from the mass.