It goes along with the "taking ideas seriously" part of [R]ationalism. They committed to the idea of maximizing expected quantifiable utility, and imagined scenarios with big enough numbers (of future population) that the probability of the big-number-future coming to pass didn't matter anymore. Normal people stop taking an idea seriously once it's clearly a fantasy, but [R]ationalists can't do that if the fantasy is both technically possible and involves big enough imagined numbers to overwhelm its probability, because of their commitment to "shut up and calculate"'