Why is it that every time single player games succeed and sequels are made, people demand a co-op? Isn't the point of the single player genre to play by yourself? Go play Call of Duty if you can't live without someone else's mic making static noises through your TV.
The biggest argument in favor of co-op/online interaction is "if you don't like it , don't use it." Yeah, just have a developer add a few lines of code so you can fart around with your buddy and sell all his stuff when he's not around.
Understand that making Fallout 4 co-op compatible would be a huge undertaking. It would delay the game, and any way you spin-it, take away from the single player experience. Not to mention the potential for bugs (like there aren't enough of those in Fallout).
If you really need someone by your side at all times, a companion is more than suitable.
And think about it: isn't part of the beauty of Fallout being able to escape the real world and isolate yourself in a post-apocalyptic Earth where limitless possibilities await? With adventure and danger and epic quests? The journey should be yours and yours alone.