I feel like there it is assumed as fact that  Activision killed Guitar Hero and the music genre at large by over saturating the market; however, I think that the music game genre itself had a built in short life span, and that from a business perspective Activision made the right choice maximizing their music games while they were still popular.  Even back to the Dance Dance Revolution days, the music genre has had a boom or bust style.  Let's say Guitar Hero Aerosmith never came out, would that improve Rock Band 3 sales?  I would say no.  I think after 5 years of plastic instruments, regardless of the number of games, everyone is just ready for a break.