Another way in which appanages could be made more attractive would be to have rulers of appanages function as extra members of the French cabinet. So if the French king has some brilliant younger son whose stats he doesn't want to go to waste, he could give that son an appanage, thereby giving himself an extra minister with very high stats. This way, a kingdom with many appanages would have a huge pile of extra ministers they could use for all kinds of specialized tasks, beyond that of an ordinary nation. Running the country can be more efficient when it becomes a whole-effort family endeavor.
This is the kind of trade-off I imagine might actually compel players to create appanages. It has to be something much better than a legitimacy bump, and maybe even improved control by itself is not enough. But having 10 more ministers than your foreign rivals might just be enough to compel them to adopt appanages themselves. And that is the kind of game balance we want.
Of course, the same high stats cannot be guaranteed for that son's heirs...