In the actual history of the world, we mostly have had monarchies. In terms of actual "sustainability" I don't think any democratic government has surpassed the longest peaceful period of a monarchy yet. I could wrong.
I'm also quite sure that the longest monarchies have outlasted the longest democracies, but I think we'd have to define the "peaceful" qualifier.
It seems the nature of hereditary monarchies helps moderate the despotism, as there is an incentive to maintain the balance for the future of the monarchic family. If they get too despotic, they will get overthrown, and that's the end of the gravy train for not only them but also all their descendants; and they have the ability to train in these constraints from birth. So my conjecture is that this contributes to the longevity of the hereditary monarchies.
