> The predictability and stability that followed from a clear hereditary principle outweighed the problems of incompetent and irrational rulers sometimes inheriting power.
It doesn't though, incompetence is more dangerous than uncertainty. If someone wants to be a hereditary head of state as a formality, then ok that is one thing. But if we look at the most successful nation in the 1000s it is probably the UK, who haven't allowed the monarch to be in the room where the big decisions get made since Charles I was executed in 1649. From that point it is a stretch to say that the monarch is inheriting power. The power to agree cheerfully with what their government tells them to do, perhaps.
The Prime Minister meets the monarch every week. It's very, very naive to assume they only talk about the weather.
The Queen was known to object to legislation that affected her personally. And the monarchy - as the head of the aristocracy, the biggest land-owner, a major influence on the Tory Party, and a private corporation with significant business interests - can always use back-channels and cut-outs to have its say.
The British specialise in this kind of indirect hinting and insinuation. It's part of the culture at most levels, and it drives foreigners insane, because until you learn the subtext you'll completely misread what's being said.
> But if we look at the most successful nation in the 1000s it is probably the UK
There is also a solid argument to be made for France.