There are a lot of ways to be irrational, and if enough people are being irrational in different ways, at least some of them are bound to pay off. Using your example, some of the people with blind idealism may get stuck to an idea that they can accomplish, but most of them fail. The point of trying to be rational isn't to do everything perfectly, but to systematically increasing your chances of succeeding, even though in some cases you might get unlucky.