Utopias are always justified this way, and the worst crimes are committed as a result. After all, if a potentially limitless number of lives are saved in the future, you can argue for sacrificing millions today. Somehow people never seem to understand that nothing real or worthwhile is down that road, and as a species we go down it repeatedly.

I think the best argument against this, is to demand that the person willing to let many die is the first to die.