My thought was that they were running out of Countries to use, so they're going to have to do this, create a new continent, or create a new series. I like this idea to be honest, and it shouldn't take them 20 years if they became successful enough and had a big enough dev team. Though debugging would be a biotch lol
We still have a ton of provinces to explore. Summerset Isles, Valenwood, Black Marsh, and Elsweyr. Basically the entire south. High Rock and Hammerfell can both be revisited in 3D. It takes Bethesda 4-5 years to make a TES game, so that's at least 15 years before we need to worry about running out. And there's other continents in lore that haven't been explored. Akavir for example.
The games are plenty big as it is. If they took any longer to make them, or made them any bigger, they'd just lose money.