Tamriel isn't the only continent on Nirn. There's also the strange land of Akavir, and some other weird places like Lyg and stuff.
I don't think they would move the games to a new continent just because all the providences have been visited though. Considering it takes them about a decade to make at least two Elder Scrolls games, then I don't think players would mind revisiting a place like Skyrim in 20-30 years. There's a lot of history and time periods to visit in the areas we've already seen. Skyrim in the 4th era may be nothing like it was in the 1st era.
