Yeah, but who get's to decide what's "benevolent" in the first place?

One man's benevolent ruler is another's tyrannical despot.

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.

Or, you could try a liberal democracy with a strong constitution and separation of powers, based on common law and strong rights of individuals.

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.