I think that this was a good attempt, but it is still flawed. As pointed out by Owencb, our intuition that it would be bad for a large underclass with a life barely worth living to exist comes from us postulating a life barely worth living, then adding in the harms of being in an underclass which would push it into a life not worth living. If we accept that these lives are still positive, then there is no benefit from replacing these people with a single victim. Additionally, we can get the sadistic conclusion in a universe where most people's lives are not worth living by choosing to add another person whose life is not worth living, but whose life is less bad on average.