rouge the AI factions already react to each other... At least that is obvious from the non-save/load games. That we remove a single faction is of little consequence as the game is supposed to actually feature non-reactionary regions (rebels) and dead factions. One faction out of 21 is not much, and in general only means another 2-3 'rebel' provinces.

From all my games it doesn't seem that the AI factions react any differently to us than to all the other AI factions. We are 'just' another faction. And the AI factions have plenty to react to, each other.