Activision Blizzard have around $3.5 billion in cash lying around after the merger between the two companies. Where's it going to go? Who knows. Some of it on fancy new office stationary, probably. Some on getting fair trade organic coffee sold at the company cafeteria. And some on this brave endeavour: Acti boss Bobby Kotick thinks that, between the franchise's popularity and Vivendi's ownership of Universal Music, Guitar Hero as a platform could someday become a music download service to rival iTunes. Sounds fairly ridiculous considering said service would be for a game using plastic instruments rather than one allowing you to buy music for your music player, but whatever. It's your $3.5 billion, Bobby.
Guitar Hero has achieved the success it has because it hasn't been competing with iTunes. Guitar Hero is still a game, not an MP3 platform. Many people would never listen to many of the songs on Guitar Hero without the gameplay.
Elite Beat Agents is a fun game but most people don't choose to unwind by listening to YMCA. The gameplay immersive enough to override any music elitism people may have.