There are some mods (CKPlus) that solve this problem of big realms being far too stable really well.
Aye, CKPlus isn't shabby at all if your goal is a more organic, fluid state of nations/preventing super-blobbing. Unfortunately, I'm not a huge fan of the modmaker's method for achieving, which is basically nerfing the levies of Kings/Emperors. It just doesn't feel right that the Shia Caliphate can only bring up 3k troops. It does work though, and at the moment I've got a game going as the King of Nubia and I've managed to last a few years without being overrun. That, and it also has a few other features I wish I could flip back (like his removal of attrition... just because the AI is too stupid to deal with it doesn't mean it's a game mechanic I still have to work around).