It only feels that way because there's nothing to do. It already takes dozens of hours of the same mindless crap to do a world conquest (the only meaningful goal that doesnt just amount to prematurely ragequitting at some arbitrary point). Making the campaign any longer than this is just ridiculous, it ensures only no-lifers who spend all day grinding like an MMO will get to finish the game.
As a sandbox game though, there isn't a 'finish' to the game, only when the player decides that - but yes, given there's nothing but conquering, it's only that option and why the late-game and kingdom elimination is the issue (and overall lack of challenge).
I think you actually should be able to power through a campaign if you play well enough. Too many strategy developers now are scared of people "completing" their game in less than 24 hours so they add tons of anti-snowballing roadblocks to nullify the effect of good strategy, effectively just making their own game worse so that everyone has more or less the same experience. Paradox games are all like this and they are not fun to play at all.
I'm all for powering through to late-game (ie conquer full map) but they have to design the game to work around that, like better 'conquer' mechanics, better battle scenarios, smarter AI, tactics control, further troop variations, longer battles, better reinforcement waves, better siege defense/attack elements, etc...All things to enhance the battles and fights, so even if you need to take 40 castles, they aren't all the 'same' grind as it is. Instead, they split their resources also to the 'features' like the generations/marriage/smithing/caravans/workshops/alleys, etc...which, are wonderful to have for RP/world building but as they are currently, only just barebones so again, 'not much to do' there either.
Instead I think the game should be difficult strategically. So the campaign is fairly quick to complete, but you are also in serious danger of losing in some scenarios, forcing you to start again with a better strategy. And currently the campaign is so long that most players just quit when they get bored and restart anyway. Plus the developers can't add anthing that would make you lose or restard (minus permadeath which is just glorified RNG) because it would be a waste of dozens of player hours. The problem though is that they seem married to the generations gimmick, which is forcing them to make the campaign long enough to justify it. But I think the campaign is simply too long.
I've only bothered with the campaign early on - it's really not any 'different' from sandbox; just an extended prologue tutorial and annoying 'quest' pop ups. The endgame is the same. As it's designed, it can't really work for restarts given the current grind. It could be done as a fast-forward type - you gain skills quicker, age mechanics quicker, perks tailored/more effective for quicker pace, money balanced accordingly, etc...so a 40-hour playthrough should
be 'common' enough to own majority of the map by then (besides additional player-made handicaps); but reflectively, you're already controlling your grandchild at that point or whatever.
The issue is they are trying to appease both sides; but in the end, not satisfying either.
I agree that the generation gimmick was not necessary at all (and the birth/death/clan replenishing aspect needed for that), but it speaks to the intention that playthroughs could potentially last years/generations. Which, arguably, they can, but there's no added gameplay behind it even after just 20 yrs in. It's a shallow feature as it is; the education they added in 'helped' a bit but that was a few mouse clicks every few hours of gameplay. Marriage for your daughters just 'deletes' all of that since you lose them, with no political gameplay benefit; they are just your clone placeholders or unlimited companions for the exact same thing as just removing cap for companions.