It must be remembered that dukes are no longer necessarily margraves, though. And that to join a duchy, your region need not connect anymore. So it's not as bad as before, when the duke of the capital would have power over the whole duchy to appoint lords, the ability to scrap the militia whenever he'd want to support a rebellion (or add more to counter it), the likely wealthiest region of the realm, AND a duke share from everyone's income. He was the richest and most powerful person.
Now, dukes can be lords of rural regions, if lords at all. And it's rather easy to sway lords to change allegiance if their duke's inept. After all, they can join a duchy on the other side of the realm now if they want. You can also make dukes out of townslands, which helps give you options when you only have 1 city, and duchies need not always keep at least one city or townsland. So eternal dukes really aren't quite as bad as they used to be.
