What if the chance of lands adding to his power is slim to none at the current rate? Coupled with the impending losses in tax gold in his city, he might not remain a happy duke much longer.... 
Oh well, its a moot point with the code shackling him either way.
Well, once a Duke reaches a certain limit for the power of his Duchy then it is time to make his Duchy well known across the world. Sponsoring famous armies, spreading his own name in the world by name and skill, hosting tournaments and seeing his goals and agendas are met in the foreign world. There are many options to keep the Dukes happy!
Joking aside, a good ruler should always keep his Dukes, as his most direct vassals, happy. Binding them with the oath of fealty is the most basic step, but if in the long run he fails to provide his Dukes with some entertainment then he should risk severe damage to his rule. It was my point from before. If let's say, a duke in Morek is not happy cause his realm has zero prospects on the horizon and his ruler refuses to do anything about it. This results in nobles leaving and therefore making his own duchy much weaker. So, his options are to either dethrone the King or make a secession. In the long run, every Duke is probably a King wannabe. We would have much more livelier islands if this was a certainty and not just a probability.
