Go back far enough and every monarchy started with a revolutionary usurper. In France, both the legitimists and the Orelanists (dubiously) claim descent from Charlemagne. But Charlemagne’s own family were usurpers - aides to the Merovingian dynasty they usurped. Go back in any line and you’ll see thugs with clubs, not any kind of enlightened rule. I’m having trouble seeing a principle other than “it was a long time ago” that makes dynastic thuggery by one family ok, but wrong when done by say, the Clintons.
The truth is this. Take the worst politician of today and tell him: "You keep your role forever. Then you pass it on to your children. Only a popular uprising might eventually topple your rule. Go govern." The result will be positive. Some will succeed and some will fail but all will improve. Why? Because there will be no need to sell government services to the electorate. The renter will become an owner. It is, for example, unthinkable that in peacetime a monarch will accumulate government debt, -- because it is his own children that will have to pay it.