A benevolent dictatorship is still a dictatorship. Power corrupts and absolute power corrupts absolutely. So, before very long a benevolent dictatorship becomes bad. The point is, a benevolent dictator, if he were truly benevolent, would not be corrupted by power. That's the whole point. Lucas isn't saying that we should take a dude who's pretty good for the most part and give him absolute power because that would come to a bad end. I think Lucas is merely saying that a "benevolent despot" would be the ideal ruler if it were realistic.