The problem, in addition to how easy it is to avoid realm scale rebellions provoking individual revolts before, is the kind of things vassals unhappy with you usually ask for, even when they do form strong factions or revolt :
- lower crown autority -> doesn't do anything against your blobbing at all, once you are strong low crown autority may even help you expand faster as strong vassals can start foreign wars ignoring realm level truces and so may expand the realm more than you can with strong autority ; also helps you avoid more revolts with bonus relation
- establish elective monarchy -> by the time you can be described as a blob you have hundred of people in your dynasty many of them landed, making easy to find one that is an acceptable candidate, and likely even better than your primogeniture heir
- establish partage -> mildly annoying demand (...only if you have several childs), will still end with your main heir getting the better realm, and claims on others to reform the realm as soon they have a succession crisis or their weaker realms get invaded, usually only slows blobbing for one generation
- deposit current ruler -> unless you have no heir it's not a problem
- independance -> usually never happen if you distributed lands correctly as only fully non de jure vassals seem to ever ask for it, rebel alone instead of in a realm scale rebellion and so are easily squashed ; and even if it happened, as you usually put vassals of your culture in command, and unless neighbour realms are same culture, they will fail to diplo-vassalise the new independant dukes, making them easy to reconquer pushing some claim of a family member that stayed in the realm after giving him a de jure title
- deposit you in favor of a claimant out of your bloodline -> the only really game threatening thing, but you can see it coming just looking at the list of pretenders and easily make sure dangerous non bloodline claimants don't remain free to push their claims, as vassals very rarely support one who isn't already part of your realm (the only case where it'd really a dynasty threatening problem is a faction supporting a strong foreign ruler to take the throne, but it's so rare I only had the case 2 or 3 times in dozens of games played to the end ; and also the foreign ruler doesn't side with rebels and invade, nor faction join his claim war if he starts one, they may have the same goal but they act independantly) ; also you'd remain one of the strongest vassal of the realm even if it happened if you correctly developped your demesne
There's just nothing likely to cripple an huge multi kingdom in there. Partage may make the next ruler spend his life fighting his brothers to reconquer previous lands but that's all. Lower crown autority or elective monarchy usually even make a big realm more efficient.
Now if rebellious vassals started to :
- support foreign claimants when they try to take your realm (aka don't start revolt for him, wait for the foreign claimant to attack and join his war)
- directly pledge to a neighboring realm they like the ruler better, based on mutual relations only (instead of needing to fight for independance first and then de jure and culture factors making such pledges very unlikely)
- ask for seniority or ultimogeniture to make you next ruler weaker (if they detect it'll mean a senile 90 years old, or baby will have the throne, leading to a vassal controlled regency)
- instead of removing you after a rebellion force you to appoint a regent who will always oppose arresting the faction members and only allow to give conquered provinces to faction members for the rest of this reign
- change gender rule of inheritance law so an already married woman (likely the wife of faction leader) inherit
- ask you to reduce your own demesne (especially give up the ultra rich capital county to capital mayor, think Etienne Marcel)
- ask you to distribute large sums of gold / special highly remunerated honorary titles among them (and/or to multiply the salary of all existing honorary titles), creating economic problems for a long period
- ask you to break key dynastic ties (ie repudiate your wife they hate, what will unfortunately piss the 500 provinces Byzantine Empire she secured a non agression pact with, or lead to your excommunication)
- ask you to declare your most capable heir bastard (there's always some rumor to justify that)
... The vassal threat would be something completely different.