Many things can have happened in a millenium (ESO takes place 1000 years before Skyrim's course of events)
QFT. If TESO will ruin the next ES it will be in another, more subtle form: diverted resources which could be better spent on developing a bigger, better single-player TES.
If I want to go on an epic adventure, I want to be the only one doing it. I don't have to worry about strangers coming up and killing me "for teh lulz" or having to team up with them to complete a quest.TES games draw most of their appeal on setting and immersion.
For all intents and purposes it seems like TESO is Skyrim with people and a monthly subscription. This should be worrying to Zenimax as this model is precisely why SWTOR is performing far below expectations; it is just KOTOR III with other people and a subscription fee. Some people like multi-player: good for them. If the TES franchise is damaged by this attempt at an MMO, I'm going to be quite displeased.