The focus has always been on the player controlled Drake doing everything, though. Even when he was with companions, he did everything. In Nepal(where he's alone for a good portion before he meets up with Chloe), she gets stuck in the elevator while Drake does everything. In the temple, Chloe waits while Drake moves all the arms, and she moves the main mirror twice while Drake climbs everything in the other room. When it's just you and Elena, she waits while you stealth kill the people below you, and Drake gets to have the cool sign holding shootout while Elena just lowers a bridge.
In the mountains, Tenzin gets seperated with icecicles while Drake finds a gun and fights the creature, and all the bad shit happens on Drake's half of the big ice room to make him do more platforming. Stuff like that is apparent all through the game, because it always wants YOU to have the best experience and have the most cinematic time. Speaking of that, the standout moments for me are the Train and the Convoy jumping part - Both ONLY Drake does.
I don't have as much of a recollection of UC1, but I do know it was atleast very similar in that focus. Either way you cut it, forcing co-op in the campaign would change the focus so that the second player doesn't have a boring time, but that steals the limelight from player one. I'd have faith in them making it "work", sure, but it'd cause a change I don't want in the next Uncharted.