As sad as this is, it's not entirely unexpected from me. I've been looking for the GOOD Pro Guitar (y'know, the one that actually uses strings) lately to move on to that from the basic guitar, and it's impossible to find anywhere. Likewise, Gamestop has stopped carrying instruments for the most part, and you really don't see much in the way of Rock Band news unless you go directly to their website. This is incredibly sad for me that I'll never get to play songs from some of my favorite bands, but who knows, they're small enough that I might be able to get into contact with them and use RBN to import the songs directly? (Psssh, as if I were to be that lucky).
Anyway, it was a good Run Harmonix, and I look forward to seeing what you have to offer in the future. So until RockBand 4, Godspeed Harmonix, Godspeed.