There are so many problems with this.
Relativity: people do not have experiences before or after each other.
On another planet, a happy civilization with our values existed billions of years ago. Over the course of its existence, there were quadrillions of people. Everything we do now is almost worthless by comparison, because they are all bringing down the average.
Oog, a member of the first sentient tribe, which has just come into existence, is going to be hit very hard with a club. Thag, a future member of the same tribe who does not yet exist, is going to be hit very hard with a club three times 100 years from now. If you can prevent one of these, it depends on the number of people who live in tribe between now and then, even though they would no longer exist.
2 is irrelevant, because it doesn't matter whether our current utility is in some absolute sense low, because we will still make the same decisions! U(A)=1 U(B)=100 gives the same outcome as U(A)=0.001 and U(B)=0.1.
1 and 3 can be solved by taking average utility over all agents, past and future. But that's irrelevant to me because... I'm not an average utilitarian :-)
I'm waiting until I can capture my moral values in some (complicated) utility function. Until then, I'm refining my position.
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.