In either universe, the earth can only serve so many people. Since we're proposing hypotheticals, here's another one: suppose all the babies that would be born in either universe were born already, then we had to decide whether to keep the babies and kill off the old folks, or to kill the babies and keep the eldery. I don't think anyone would propose we let the older generation live. This hypothetical is in fact much closer to the situation we are in...