If the MMO flops it will adversely affect the single player games and the corporation as a whole. There are millions of dollars at stake.

Not to mention shareholders and investors will be less likely to devote the kind of budget toward another iteration of TES that Skyrim had, lessening the chance for it to surpass Skyrim in any way shape or form. You all have to remember that by and large the suits don't play the same games we do, they play the stock market and simply run the numbers. They know nothing about the difference between the MMO versus the SP game and you're only as good as your last success. If the MMO fails, that will be the latest game in the TES franchise, and confidence in the franchise AS A WHOLE being the #1 fantasy RPG franchise in the world will diminish significantly. The number guys won't be able to justify the spend quantifiably. I wouldn't be surprised if they force Zenimax to move on from TES and devote BGS solely to Fallout from then on.
Their expectations are the same kind of success when Warcraft jumped from RTS to MMO and that is simply not realistic. Their expectations will be shattered. They are saying: "Well if there's any franchise that can compete with WoW, it's TES, look how amazingly past titles have sold, and our fanbase is huge! We can't lose!" They can, and they will, badly.
Think about the scenario of a Call of Duty MMO, except that it is third-person only, with cooldown/hotkey non-interactive combat and takes place in WW1. Now imagine how many of the rabid COD fanbase would buy it. I'd go with roughly 5% as a fair estimate. You think after that, regardless of developer, Activision is gonna devote so many resources into the next installment of the COD SP/MP games? Hell. No.