That's the most likely reason it would be affected, I'd think. No new Ultima single-player games have started development since UO came out (U9 was already in development when UO came out and was a half-hearted effort at best, by the time it came out). No new Warcraft games have came out since WoW came out. I don't think anyone's expecting a new KoTOR game. There's plenty of precedent. In the eyes of a publisher, a cash-cow subscription MMO is the ultimate evolution of any particular franchise.
Differents studios, different times, different philosophies and so on…
The issue with Ultima is that EA thought it was a great idea to turn Origin into an online only game, and that turned out to be a terrible mistake. And since Origin closed though, there's been three attempts to make new single players Ultima games that I know of actually (Ultima Resurection on the PSP, Ultima Reborn on the NDS, and an AAA Ultima game with Obsidian) though the first two were cancelled during development and the later one died at the paper stage.
Warcraft : granted. But there's the issue that it's the same studio doing WoW that did the the Warcraft RTS games, and also the fact that by putting WoW chronogically after Warcraft 3 it kind put in wrench into furthering the story in a new RTS. Which is probably the reason Bethesda is putting TESO a thousand years before the Skyrim.
KOTOR : this is more a LucasArts issue really, and the fact they just don't seem to know what to do with their IP (they actually had a KOTOR3 in development internally after KOTOR2 and they cancelled it). There's actually no reason to think a new Star Wars RPG wouldn't be doable in spite of SWTOR realy - there are plenty of era in the SW chronology such a game could explore.
And as I said even putting the whole "A different studio is handling the MMO" (since I guess you make the argument that TES will stick to MMO and Bethesda Game Studios will just stick to making Fallout games), again: you're forgetting the importance of consoles gamers.
There are MILLIONS of consoles gamers who buy, play and enjoy Elder Scrolls games. There is no way in hell Zenimax and Bethesda will not want to bring more TES games for these gamers. And while you could argue they'd make a console of TESO... considering consoles MMOs tend to fail miserably, this seems unlikely.
And considering we're likely not seeing TES6 before 2016 or such, there's no risk of TESO and TES6 competing with each other and TESO could serve as a "fix" for fans while they wait for TES6.
So while there are precedant, I think the Bethesda situations is very different.
-Sergorn