• It costs a lot of money and it takes a lot of time to create a whole new game from scratch to sell.
  
• It costs less money, it takes less time to create new content in a game that is already built.
  
• Skyrim cost 29,99 € on Steam and Dawnguard + Dragonborn cost 39,98 € together.
  
• Skyrim development started in 2008, so it took about three years to make the game. (Based on the Wikipedia article for Skyrim.)
  
• It says well enough when two major DLC that was released the same year cost more than the game itself.

In my opinion, they should never stop making new DLC as long as they have the imagination for it.
They can stop with the DLC when they announces that the next The Elder Scrolls game will be coming out next year, from when they announce it. Right!

They also specifically said there was more to come in 2013.