Its not about the studio its about the name! Two games set in the same universe will compete with each other. Why would they want to compete when they re owned by the same company?
Yeah, the same way how they released countless numbers of Star Wars games while simultaneously having Star Wars Galaxies up and running, and how they have LOTRO while simultaneously releasing new LOTR games? With the former example, they were actually able to release many best-selling games set within the Star Wars universe, including the KOTOR games, two Battlefront games, The Force Unleashed, the LEGO Star Wars games, and who knows what else. Besides, they would be crazy to
not do another single player TES game for several reasons.
First of all, there are a lot of complaints that an MMO might mean the end of traditional TES titles.
Secondly, Todd Howard is passionate about making the single player titles, and if Zenimax decides that he shouldn't make any more TES games in the traditional single player design, they could lose him, and that would be bad because he is one of the key members of the company that keeps them afloat.
And finally, THEY STILL HAVE THE ALDMERI DOMINION THING GOING ON! That isn't going to be a story arch that will be polished off well in DLC alone, and the MMO is certainly NOT going to deal with the Aldmeri Dominion, so naturally, they would have to continue with that stuff in new single player installments. They also have the modding community to worry about as well, since MMOs don't allow the same level of modding as singleplayer games do.
These are some of the reasons why they will need to continue developing the traditional single player titles. Should I also mention that there are things they can do in the single player games that they likely won't be able to do for the MMO, such as vampirism and Lycanthropy like they do it in the single player games? Unless they can find some way to work that stuff in logically, they'll also lose fans that way if they choose to stop making single player numbered entries.