This brings back the original problem of killing people to bring up the average.
That can be dealt with in some usual way, by setting the utility of someone dead to zero but keeping them in the average.
It feels odd to take an average over a possibly infinite future in this manner. It might work, but I feel like how well it matches our preferences will depend on the specifics of physics.
EDIT: This also implies that a world with 10 happy immortal people and 10 happy people who die is much worse than one with just 10 immortal people. Would you agree with that and all similar statements of that type implied by this solution?
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.