Or if everyone plays their current nobles more active rather then being drones, the game becomes more fun and thus has more players? Adding more nobles will only have a negative effect on this..
This is a good point. How many characters can one person play, and play well? Three characters is relatively easy. Four can be difficult. But actively playing 5 noble characters, and devoting all the effort to them that is truly needed to make them contributing members of the realm, and not just travel drones? That's rough.