The only game that probably should be compared to TES would be Two Worlds, which was always too ambitious for the developer who tried to pull it off. As a result, it went face-forward into the ground.
The only true competition BGS has is really itself. That could change in the future if anyone else decides to do an open-world FPS RPG.
This is my theory. The thing is...people noticed the ambition that TW had. They forgive it's flaws because they can see what the developer was trying to do. Now, I couldn't get passed some of the stupid annoying mechanics of the first, but I'm ready to give the second a try. It's not as wildly successful as TES, and it's turned a lot of people off. But it has a growing fan base, and with it, a growing power to realize it's ambitions.
So if they or another game continue on this arc, and the Elderscrolls fails to keep pace (Oblivion took one step forward and two steps back) then it may see itself eclipsed in the future. I think Skyrim will be an improvement from Oblivion and amazing in general, but there is a chance that it will be the last "dominant" ES game, before other companies learn to emulate (if not improve) the open world open story model, while at the same time retaining all the positive traits they had to develop when they couldn't compete in that arena.
Now there is a new AAA open world rpg coming to challenge mighty Beth's open world rpg dominance... Its Kingdom of Amalur. Its got a big budget and an all star team behind it, including ex Bethesda employees.
I briefly looked at that game. I won't get into how much I dislike RA Salvatore, because plenty people love him, and while Ken Rolston has done good work for Bethesda, his record's not unimpeachable. Also, dude who created Spawn and some pro MLB player whose also a game dork. Meh.
All I will say is that the game kind of looks like a single player, graphically impressive WoW. Somewhat cartoonish and over the top. I'm sure it will make millions.