As I read these comments and recall my own experience I think I see where the problem is. Dev's are willing to prevent unrealistic world conquest, but they are doing it in a sort of wrong way. Well, having social studies education I know that to prevent some behaviour of people (i.e. youths) by forbidding it or putting to stop is much worse stategy than giving them something else to do. Sport, culture and entertainment in fact do more for crime and prevention than strict laws and police patrols.
And where the problem lies, is that after you carve your Kingdom/Empire, kill some rebels and lock troublesome vassal into dungeon, you are left with nothing to do except starting next conquest. There are too few things to do in peacetime - and I don't see bulding next tier of barracks or sending generic gifts to all vassals with negative opinion as 'thing to do", jthese are just a sort of grinding... Education and council are steps into right direction, but too small steps. More interactions with dynasty and courtiers, more plot options, more adventures and events, less linear and more overlapsing with each other, that would be the right thing. At the moment, after few succesions each rule tends to mirror the previous. In my case: find a bride with traits, marry her, family focus to groom a heir, switch to stewardship to become more competent ruler and... what? Conquest, because you have nothing else to do. Sometimes rebellion or troublesome courtier who seduces wives, but you just keep getting the same stream of the same events, and after fourth or fifth ruler you even don't read them or consider, cause you know them by heart. No thinking, just clicking the same choices time after time, meanwhile having nothing to do just fabricate claims and conquer, conquer...
Devs are trying to prevent countries from blobbing, and coalitions and claim fabrication did this right. But they handicapped AI, world-conquerors and sim-players likewise, giving them only scraps in return. You shouldn't sit bored on a throne for 40 years just to let your threat fall down. You should have just better and more absorbing matters to handle, places to see, people to visit, so that wars would be considered as 'waste of time of my ruler than could be spent better', not 'the only thing you are doing not to quit the game'...
Just to make it clear - I love the game, and I enjoy latest DLC, patches and even so hated coalitions and autojoined wars. But I am trying to understand why so many people are disgruntled and how can it be repaired... am I wrong?