You'd be surprised on what it takes to developed a AAA game, and how many people will be needed. I awesome Bungie is using all its' staff for Destiny since they want to make it awesome. I am sure there times when some employees are coming up with other project ideas, but are focusing on Destiny. Once Destiny is launched then I am sure they will go back to those ideas for a little bit to see if they can implement them in the DLC and/or future Destiny games.