I think the answer to this question is simple: the power vested in the state should be exactly proportional to how much you can trust its leadership. Theoretically, any system can be corrupted by stuffing it full of enough bad people, and any system can work if stuffed with enough good people. Ideally, the best governmental system would be a totalitarian system (hear me out) led by an infallible despot. However, since people are very, very fallible, despotism invariably fails. So, you just cut back on their power and make them a bit more accountable, and eventually you end up with a constitutional republic.