Although philosophers have explained variously the correlation between simplicity and truth, they generally agree that simplicity signals truth. Unless utilitarians can otherwise justify it, searching for a simple moral theory means searching for a true theory.
All searches for truth are searches for simplicity
Utilitarianism is a search for simplicity
Therefore utilitarianism is a search for truth
While I understand that utilitarianism being a search for simplicity is evidence that it’s a search for truth, that does not give you license to automatically assume the worst of a theory you dislike.
This is the fallacy of the undistributed middle. You say, essentially,
All searches for truth are searches for simplicity
Utilitarianism is a search for simplicity
Therefore utilitarianism is a search for truth
While I understand that utilitarianism being a search for simplicity is evidence that it’s a search for truth, that does not give you license to automatically assume the worst of a theory you dislike.