And then eventually the heir happens to be some dipshit. Not that the dipshit rules. Invisible power structures (IYKYK) emerge, and the King’s “master of stool” is suddenly the most prestigious job in the Kingdom. Eventually the actual rulers want to be recognised as such, and violence ensues. Look, there are no “good” systems. It’s a space of trade-offs where you pick your poison. Or, well, it’s only a democracy that lets you “pick” the poison. Arguably, the only real benefit a democracy provides is non-violent transfer of power. There are “democracies” where even this is not a given, and then yeah, a monarchy would be better, as the cycle of violence is longer.