I think Bethesda like Rockstar is one of those special companies that can get away with releasing new titles so infrequently. GTA V has made so much money they could easily afford to spend the next 20 years developing their next game if they wanted haha. I hope they never try and "Call of Duty" it.
I loved New Vegas, Obsidian had such a good base to work off tho with Fallout 3. Not sure I'd trust them to create Fallout 4 from scratch!