That would be an improvement, but then any Roman Empire game would run into a similar problem with Tunis and Alexandria. Likewise if you'd like to go after historic possessions in Hispania or Gaul. Unless the AI is successful in creating Francia or Hispania, these realms would never drift.
Considering that recreating the Roman Empire is one of the main challenges offered up by LoR, I find it odd that any game attempting to do so will lead to a majority of your Empire being outside of your De Jure realm and thus be significantly more revolt-prone.
It feels like an arbitrary limitation designed to frustrate the player and provide an artificial challenge.
It should be significantly revolt prone and outside your de jure realm. The Roman empire idea is one of the worst ideas in LoR and should be extremely hard. Justinian almost destroyed Rome trying to retake it, the mostly muslim and heretic Syriacs and Egyptians would be even less likely to welcome the greeks with anything other than a pike and a crossbow. These are parts of the empire that welcomed the muslim arabs as liberators because of greek fanaticism against their branches of christianity.