And while I understand your point about Mooney being defeatist, I think it's a matter of degree. Politics (and good government) is definitely about "the art of the possible." The reason why some more idealistic/simple approaches fail (communism, laissez-faire capitalism, objectivism, fundamentalism, etc) is because they require human nature to be different than it is. Plus, the dark side of idealism is the idealist lashing out at fallible human beings for not living up to an impossible ideal or theory.
In some ways, good governing is like good design, in that designs need to take the nature of actual human beings into account. If everyone is always clicking on button A when they mean to click on button B in a user interface, it's time to redesign the UI rather than berate the user.
Still, I agree it's a balancing act. Taken to an extreme degree just accepting people's actions as they are can up encouraging a kind of lowest common denominator. Still, optimizing for actual human nature is bound to be more productive than optimizing for an ideal human being that doesn't exist in the real world.
Well, more specifically things that interfere with the liberal secondary response of tending to compensate for motivated reasoning. And yes, that does lead to more complexity, but liberals do tend to over-compensate at times (tending to always side with victim, for example).