Another analogy would be the Prince of Wales. As you pointed out, they are usually also the Dukes of Cornwall and of course heirs to the English Crown. But that doesn't change the cultural or political borders of Wales. It changes the domain of the guy who happens to rule in Wales.
And that is, in turn, exactly what I mean when I say that duchies should be seen as personal and not territorial. The Prince of Wales is the
Prince of Wales, not the Prince of
Wales. If for some reason he decided that Cornwall was enough and to give Wales to some other guy, the other guy would most probably be only the Count of Wales, unless the other guy was really really cool.
There were duchies whose title expired, that didn't necessarily destroy the cultural unity of the people living on the land.
