Not at all. It is perfectly possible that society would be better off under some benevolent dictator, but that is nevertheless rejected as unacceptable.
Rejected as unacceptable by who? Why should this be rejected as "unacceptable"? This form of government could only be "unacceptable" if was not in the best interest of the society, which brings us back to the common good again.
What you're really saying is that a dictatorship could serve the common good, but since it is not a representative form of government, it would not be serving the common good as well as a representative form of government would.