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.