Nearly all the dukes are Traditionalists, if one person secedes the rest will beat that duchy back into submission.
So, let them try. All Dukes should have enough funds to hold out for quite a while. Level 5 walls and a bunch of militia, with maybe a few knights who like the idea and it will be difficult to break the city into submission. Especially if it is one of the somewhat richer cities, of which CE has quite a few. It's better than doing nothing for years on end I'd say.
