Depends how you want to define it, i'll be more specific
Is it an Elder Scrolls game? yes
Do I want it to be an Elder Scrolls game? no
Is it an Elder Scrolls single player game? no
Do I define an Elder Scrolls game by it's single player? yes
Hopefully you can understand my opinion now. Your point about Halo may be right, but before now TES has never sold out before, or if they have it has never truly bothered me. This on the other hand does. To me this game is just an mmo, it's much easier that way. Because to me Elder Scrolls is about the single player, that is how the experience is defined. Anything else just won't be Elder Scrolls. It's just my opinon, you don't have to agree with it.
Remember that whole angle used against people who don't like Skyrim? How did it go... "TES games have always been changing..."
This is a TES game, one hundred percent, but with a different focus, that focus being having thousands of people play together. Just like Skyrim had a focus to get thousands of new players playing.