The ONLY way they can really break this game in the long run IMO is charging too much for the song downloads later on. This is assuming that it becomes general knowledge to the public that Harmonix is making this game, and the single-instrument content is as awesome as GH1/2's was. It sounds like it will be, though.
Even if the peripheral cost is high, if the song DLs are cheap enough I know I'd suck it up with my friends and eat the cost. All in the name of endless hours of pure distilled fucking awesome. The song list is basically unlimited for a video game.. if anybody can get the songs, EA+MTV can. I mean, they got Metallica... and a 'substantial number' of their songs?!
I'm just imagining getting drunk with my friends and playing Blackened. Jesus.