@ both. It was my understanding this conversation was relatively new, and that this was a forum for suggestions.
I'll reduce archd's entire comment to "Trying to keep the game simple", because when TW decided to create Bannerlord they said to themselves, "Lets give some guys some bows, some guys some armor and we'll just keep it real simple."
I'm sure that when developers said "Hey let's listen to what players think about how we should implement a Co-op Campaign", the answer they were looking for would be, "It's a problem for future TW developers."
It seems to me that the co-op play archd describes already exists in quick battles, except for not being able to use personal heros, and play for the same stakes as in the campaign. While it may be fun to join in to gameplay at all the most exciting moments, it won't do anything for the evolution of overall gameplay(which is what I imagine developers are looking for in these forums).
How are you going to call it Co-op, when you're asking the second player to just wait on stand-by for a battle to maybe happen? Finding a fluid game platform for a campaign that can support single or multiplayer and switching back and forth would be like the most epically complex MMORPG but with the smooth plug and play ability that Nintendo Switch was supposed to have.