Not to mention, if we want a multiplayer system that involves more than spamming attacks at your enemies, you have to have clear roles. It doesn't have to be "tank, heals, DPS", but it has to have some structure. Boss fights need strategy, each role needs a purpose. If you can make an almost infinite number of "classes", then how do we establish good strategies for defeating bosses? And if there's no proven set of strategies to defeat a boss, then they'll honestly just become what they are in Skyrim - generic enemies with a crap-load of stats.
The Elder Scrolls relies on a dynamic world. I kill a merchant, not only do they stay dead, but someone just might take over their shop. You can't have this kind of freedom in an MMO, or else all of Tamriel would be devoid of NPC life by the end of the launch day.
A good MMO needs instancing of some form. Because then, you either have
a) a set of well-crafted dungeons that you can only visit once

It seems to me like people want the MMO to be a Tamriel-wide single player game with a truckload of other players. A successful MMO (I'll say it, World of Warcraft) forces you to cooperate with players, fighting through failure, to reach end-game content. A poor MMO focuses on a brilliant story and amazing graphics, which effectively turn the game into a single-player game with a subscription and a chat system. If you want a truly successful TES MMO, it has to be one that forces multiplayer upon you.