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The main thing is that they were only barely part of France anyway. They were essentially independent rulers whose recognition of royal authority was mostly nominal.

And of course, within a few generations, the Duchy of Aquitaine did break away (as Eleanor of Aquitatine dumped the king of France and married Henry II of England to form the Angevin Empire, where parts of it would remain until 1453). Meanwhile, Toulouse didn't go quite as far, but it did cultivate strong ties with the crown of Aragon, until it was conquered by more reliable northern French lords during the Cathar Crusade.

They weren't alone in that, of course. William the Bastard, before he decided to invade England, had already established a formidable military reputation beating up on his neighboring French vassals and even the King of France, Robert of Burgundy had basically forced the previous French king to hand over his duchy, and Flanders was a perpetual bone of contention between France, the HRE, and later England.
They were de facto independent but CK has to tread the lines as a game. Should the Normans be papal vassals at game start and even when they have their kingdom eg.
Aquitaine, burgundy, and Normandy are sometimes seen as principalities in histiography for the level of autonomy they held, but should they be independent or not.
The return of tributaries in a future dlc will be very useful to help show the level of autonomy that some had, even if its unlikely to update bookmarks

Speaking of swearing fealty to grow more easily within, I find it really annoying that Hapsburg start as counts with imperial immediacy rather than vassals of swabia, as it means they're unlikely to grow to anywhere near their historic size in their first iteration
 
It was too strong or too generous in CK2 and I still think it's the same in CK3. There were some weird cases in CK2 - you could load up 4th crusade start date and reunite Byzantium incredibly quickly with "offer vassalage" because of de jure status (only a few bribes required here and there). In CK3 this situation (if there was 4th crusade start date) would work out almost identical.

I would personally nerf it, maybe not by adding flat minus chance but by adding more variables and requirements. Maybe some events around it?
I mean we're not going to get the historical complexity of epirus being added back into the empire so gold and offer vassal with de jure of same religion same culture is all we can have
I think the difference is that CK3 makes Aquitaine a de jure kingdom, which makes sense in 867, but was quickly integrated into France in real life, and had faded from consciousness by 1066. Thus, dukes of Aquitaine would have been more concerned with securing their autonomy (low crown authority, favorable vassal contract), rather than with securing independence from an illegitimate liege, which is what CK3 models.
Maybe aquitaine should be scrapped, with it all being de jure France in 1066, but the decision to reform aquitaine not being that complex requirements. Maybe being allowed to add Barcelona back in automatically if you own the duchy