Another thing that's pretty crucial here is that rationality is only aimed at expected winning.

Suppose we live on Lottery Planet, where nearly everyone has a miserable life, but you can buy a lottery ticket for a chance of $BIGNUM dollars. Nonetheless, the chances of winning the lottery are so small that the expected value of buying a ticket is negative. So the rational recommendation is to refrain from buying lottery tickets.

Nonetheless, the agents who would be "smiling down from their huge piles of utility" could only be the ones who "irrationally" bought lottery tickets. (Credit for this example goes to someone else, but I can't remember who...)

You shouldn't expect rationality to help you win absolutely. Some people will just get lucky. You should expect it to help you do better than average, however. The rationalist on lottery planet is certainly likely to be doing better than the average lottery-ticket buyer.