I mean a normative theory (or proposal if you prefer). Utilitarianism clearly fails as a descriptive theory (and I don't think it's proponents would generally disagree on that).

A normative theory that proposes everything would be fine if we could all just agree on the optimal outcome isn't going to be much help in resolving the actual ethical problems facing humanity. It may be true that if we all were perfect altruists the system would be self consistent but we aren't, I don't see any realistic way of getting there from here, and I wouldn't want to anyway (since it would conflict with my actual values).

A useful normative ethics has to work in a world where agents have differing (and sometimes conflicting) ideas of what is an optimal outcome. It has to help us cooperate to our mutual advantage despite imperfectly aligned goals rather than try and fix the problem by forcing the goals into alignment.