Or *all* in a single duchy, which I predict will be the trend.
Exactly. The benefits are obvious:
1. Everyone gets their tax income in gold rather than bonds when in-realm.
2. All civil work and police work gets the in-duchy penalty rather than the harsher out-of-duchy penalty.
3. All the region lords can see status reports realm-wide instead of just for their duchy.
Even those who oppose realm centralization will find it pretty hard to argue against those benefits.
Basically, by allowing players to draw their own duchy borders, you have effectively destroyed the duchy system. Duchies will just be realms now.
If the intention was to roll back the duchy system, we should just do that. You're not really giving players additional freedom or choice when there is an obvious, optimal configuration. Everyone will scream and shout until their ruler presses the "make my realm not suck" button.