My short answer is that ‘neutrality against creating happy lives’ is not a mainstream position in the EA community. Some do hold that view, but I think it’s a minority. Most think that creating happy lives is good.
I agree with this answer. Also, lots of people do think that temporal position (or something similar, like already being born) should affect ethics.
But yes OP, accepting time neutrality and being completely indifferent about creating happy lives does seem to me to imply the counterintuitive conclusion you state. You might be interested in this excellent emotive piece or section 4.2.1 of this philosophy thesis. They both argue that creating happy lives is a good thing.