What about when you think of Dragon Age or Mass Effect? You (used to) think of interesting characters, non linear story, six...
you get the point.
So what effect will this MMO have on the ES name? On the ES reputation?
Judging from what I've seen of the MMOs in the past few years, it doesn't look good.
Now concider the alternative.
If it is successful how much of that success will carry on to the rest of the series? Elements that may be good for an MMO but would be disasterous in a SP game.
You cannot say that this game will have no effect on the main franchise. They bear the same name and thus cannot be seperated so easily.