It's a bit different. For the imperial side, a defeat should mean a defeat - with the remaining forces and holdings joining one of the remaining imperial pretender factions which goes somewhat in the direction of
except that it seeks to limit the scripted behavior and utilize the general sandbox mechanics - which is probably also the reason why you (sometimes) can see a new declaration of war after "defeating" an anti-imperial kingdom. It ends the scripted, enforced war but returns the factions to the general sandbox mechanics. (IIRC, though, if you are king of your "supported" imperial faction, you can reject the peace offer of the barbarian faction and end them for good... to avoid any such "backstabbing".)
I appreciate that TW is trying to do a good thing by avoiding excessive scripting, but is a little scripting actually bad for campaign mode? Wasn't the point of having a campaign to create a more structured path through the game?
You have a problem in Bannerlord right now where a lot of people are saying the endgame drags on, for two main reasons. Once you get to 40% of territory taken, everyone constantly declares war on you without any pacing, without any breathing room, for a long, repetitive slog of battle after battle after battle after battle after battle after battle after battle after battle. Battles lose their sense of uniqueness because they lack tactical depth or balance and faction troop trees feel too samey, but even if TW fixes those issues there are just too many battles needed to complete the game, and most battles only have minor impact on the enemy's ability to war.
Then eventually, once you get to about 60% of territory conquered, there are no more credible threats. The game loses all challenge, but the enemy still doesn't realise they're beaten. So in order to get your endorphin hit for painting the whole map you have to sit through a prolonged anticlimax. (This problem applied to Warband as well).
Wouldn't it be more satisfying and fun to condense a lot of the lategame battles into one big challenging battle which wins you the game, and skips that boring last part when you win it?
A climactic moment where you break the enemy, like the Battle of Mankizert, the Battle of Tours, the Battle of Marathon... Even Bannerlord's backstory has a huge climactic battle, the Battle of Pendraic.
Considering Sandbox mode has completely unscripted gameplay, I think it wouldn't hurt to add a bit more scripting to the Campaign mode to make it a more satisfying experience.
With all this said obviously getting the sandbox working right is the much higher priority.