>in it's less acceptable form would be "slavery would be fine if slave-owners were just nicer to their slaves".
Sure, if we're thinking in a vacuum. Even if we go religious, what's the difference between modern prison and the Garden of Eden? It's part of why we call tech environments with a lot of amenities a "walled garden".
Digression aside, we have millenia of knowledge to know that never ends well and millenia of knowledge to know that slavery is never about trying to treat people well to begin with.
>All are a from of argument that some people should be be placed in a position of irrevocable power over others (by accident of birth into a privileged race or class) ... and that's all OK if they treat their underlings well
In theory, yes. And like slavery we have millenia of knowledge on how NOT to run a country.
But unlike Slavery we have seen that the alternative to no government ends worse. So it's an unfortuante compromise needed for a society in that someone needs to make widespread decisions. The more modern (but not necessarily best) methods to prevent corruption is simply to make sure a person is never in that seat of power for too long.