They don't change the way you play, it's just more of the same.
NG+ in Dark Souls is also more of the same, just with higher stats. Should it not have existed? Should any "endless" optional mode in any video game ever not exist?
If the challenge is high enough to keep the player engaged - fighting half of your realm or more is a pretty solid challenge -, then "more of the same" as post game content is fine.
It's no different to having your save corrupted and having to roll back several hours.
The difference is you're using your heirs and extended clan members geared up and leveled up, roleplaying as an enduring dynasty, while you're waiting for the civil war or invasion to occur you're strategically building all of Calradia to resist where the threat might originate, and you're fighting increasingly more difficult challenges originating in different parts of the map.
Rather than losing everything the player has worked to gain since then in a save rollback, you're still keeping your clan members and their stats you've built up, your armour and banners and weapons and inventory, your fief upgrades, your policies, and probably half of your nobility and their owned fiefs.
Also it doesn't break immersion like a save rollback would. Instead, it fits nicely with the story. Your clan has conquered Calradia, now how long can you hold it together?
When the game has no more tricks up it sleeve it should end. Continuing beyond that in service of the generational feature is going to harm the rest of the game.
As already said:
"In scenario B, players conquer the whole world, they get told "you win", and then they have an optional objective which they can pursue afterwards which
does not in any way detriment the gameplay experience of people who just wanted to get to the "you win" screen."
If you think the game should end at the you win screen - a perspective I totally get - that's fine. You are totally capable of stopping playing when you win. Doesn't mean other people shouldn't have an option because you personally don't like the concept.