What exactly prevented this in the old system?
In the old system, duchies were tied to cities. It was impossible to have a duchy without a city. (I'm considering strongholds as cities for this purpose.) If your realm had one city, you had one duchy. Since the last city can't swap allegiance, it was impossible for the duke of a one city/duchy realm to switch allegiance. You had to have at least two before you could pull something like this.
