New GH games don't cannibalize the sales of old games. They enhance them. People coming into the franchise for the first time will want more songs, and the easiest way to get them is to buy an older game.
Gamestop has GH2 (game only) listed to be coming out the same day as GH3. The fact that people can come back a week later and pick up a whole new batch of songs without buying a new guitar as well will be huge. It happened for Guitar Hero 1, there's no reason to believe it won't happen for Guitar Hero 2.
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New GH games don't cannibalize the sales of old games. They enhance them. People coming into the franchise for the first time will want more songs, and the easiest way to get them is to buy an older game.
Gamestop has GH2 (game only) listed to be coming out the same day as GH3. The fact that people can come back a week later and pick up a whole new batch of songs without buying a new guitar as well will be huge. It happened for Guitar Hero 1, there's no reason to believe it won't happen for Guitar Hero 2.