Except you won't really be heroes, you'll be random faceless nameless warriors or mages or whatever. You won't be the Dovahkiin/Nerevarine/Champion of Cyrodiil or anything similar.
And while it won't effect previous games, to say it won't impact or effect future games is foolhardy. They are going to create content for this MMO, lots of content because it's an MMO, which means that I won't have a chance to experience that content in a single player game. Molag is the main villian of TESO, so what do the think the chances are that we ever get a single player game with him as our main villian?
To say this won't effect the single player is simply not true. They are going to retcon stuff and change things all over the place until it's an unrecognizable mess.
You cant say weather it will effect the single player game or not,you can't read the future or can you?
Nope, ill be playing alongside my RP guild in a world based on TES,plenty of heroic adventures to be had with my guild and groups.As a RP in MMOs i get involved and we actually live the game and have organised events and rewards,go play LOTRO on the RP servers and you will see what i'm getting at.
Again,you can play your single player TES and we who look forward to living in a TES virtual world can enjoy TESO.
No one is forcing you to follow this game or play it,if you aint interested then move on,simple solution really.