That's a very interesting point. Perhaps, to ensure that a range of ethical frameworks have been taken into consideration (each with their varying ideas of what benevolence really means) a more equitable situation would be a plurality of utopias each with its own ruling AI? And free movement of individual humans to whichever aligns best to their own personal code of ethics.
If such a system was incorruptible, constructed such that it would be impossible for a small group of individuals to hoard the majority of the global wealth, and guaranteed a pleasant and equitable standard of living for all - then it might work.
Unfortunately, the current implementations of this ideal fall far short.
