The game is 5 years in developement. Before Skyrim was even made.
Skyrim came out just fine. This won't touch TES6.
Again, using logic. Game engine, character face/body design, weapon/fabric/texture design was 90% of that time, all of which had nothing to do with what Bethesda was doing with Skyrim. Initial storyline to create the game world did not affect Skyrim either, because the lore already existed for this time window they created for the most part.
Now...MMO's need new content. That means eventually they will need new creatures. New dungeons, some of which will have history. Main bad guys in the dungeons will need lore and background. New 'plots' involving world dangers (think expansion packs like WoW) will need to be developed. All of this will logically have to be completely new ideas in order to keep subscribers paying. Again, I reiterate- Bethesda writers will have to be involved at some point of this process.
Now, you can say that this won't affect the
timeline of a new TES game, and I cannot fault that, for we have the example of Skyrim. But I do posit this: it will affect the
quality of a new single player game, and here's why: those ideas they are using for the MMO, new dungeons, new lore, new bad guys...are ideas that could have been used in a new single player game. Over the course of five years' development for an Elder Scrolls game, we've heard on the podcasts how little details are dreamt up and put aside for use in the game. Now, they will have to share some of those with the MMO.
How can you dispute that?