Where does it say that? (The Elder scrolls is Eternal??)
It says it in: The game was set out with a original history, very descriptive yet STILL open ended. About how the world more or less came to be, to a description of all the gods, races, religions, wars that had taken place in the first 2 eras. The 3rd era is where in Oblivion you spend alot of your time in, adding onto this story, and throughout the game, things unravel as you play, and all of a sudden Tamriel's emperor is lost..
If you look back at the old history, their was war everywhere when their was no emperor, and cultures divided. The empire had them unified, but with no emperor or heir, anything could happen. So taking into ALL THE EVENTS that Especialy, but not JUST (Arena/Daggerfall along with TES Travels contribute a bit to the base story line of the game), Morrowind/Oblivion have left amass issues with Tamriel in multiple ways, with you being right their to see it, then hearing stories of it throughout the game, and reading (The pocket guide), you see what is going on.
Its a video game that has a amazing storyline and plot which is highy detailed in all the games events, througout Tamriels and/or Mundus's history, it is still Open Ended, and much is untold. Unless they purpously wrap on this series/world into a Happy Ending in the next elder scrolls, (Which by the way would be pointless, the whole world doesent end in a happy ending just because, people/culture change always), this game could go on forever. Maybe.. further exploration into the Sea of Ghosts would lead to the discovery of more lands or something, or Gods could turn on the citicenzs for whatever reason you just dont know. The only way this game could ever end is if Bethesda stops making it and no-one else takes up the job of continuing the series.
The game is a world/story of its own.