Three characters, one game, six huge heists of the back of the greatest GTA mission in Three Leaf Clover; It would be silly not to wouldn't it? Well Rockstar aren't too fussed about how silly you think they are, it just isn't happening.
But with the Multiplayer being 'compelling' and important to Rockstar, I wouldn't worry too much about the lack of all things coop when it comes to the single player. Plus there is also DLC which Dan Houser (writer of GTA V and all that have come before) has been quoted as saying 'we have some great ideas for'. Let's just get the game out first, aye, Dan?