From my perspective, there's no contradiction here--or at least, the contradiction is contained within a hidden assumption, much in the same way that the "unstoppable force versus immovable object" paradox assumes the contradiction. An "unstoppable force" cannot logically exist in the same universe as an "immovable object", because the existence of one contradicts the existence of the other by definition. Likewise, you cannot have a "utility maximizer" in a universe where there is no "maximum utility"--and since you basically equate "being rational" with "maximizing utility" in your post, your argument begs the question.