I guess they are following the money trend I wouldn't be surprised if they continue on this way although this does mean that multiplayer has to be top notch or pull a battle arena (I really don't want to see that).
Alternatively they could also just release the game with a promise that they will deliver the campaign mode at a later date (hopefully with co-op).
I remember when overwatch released without a story mode (single player) and it was a huge success but I agree, we all love abit of lore in any game we play so bringing it out slowly would be a great idea if they initially didn't plan to from the beginning!