That nature is (always) a better reasoner than man isn't a credible premise, particularly these days, when the analogous unconditional superiority of the market over central planning is no longer touted uncritically.

Do you assume individual rationality's justification is utility maximization, even if we settle for second-tier happiness in proxy? Programmed to try to maximize happiness, we act rationally when we succeed, making maximizing utility irrational or at least less rational. Utility has nothing more to recommend it when happiness is what we want.

Another way of saying this is that happiness is utility if utility is to play its role in decision theory, and what we've been calling utilities are biased versions of the real things.