As a counter-argument, RB3's engine can be the template for RB4, which means that little has to be done on the development side of things. That's a significant cut to costs right there, and if they re-use art assets in RB4 the way they did with RB2, the majority of money they need to concern themselves with is in licensing. All the money on new stuff was already spent on RB3; RB4 would be patching up the issues people had with RB3, plus new music.
While true that the sales are pretty far down from RB2,
this chart (which, yeah, do not take as gospel) shows that it appears to still sell fairly steadily. Also, Columbus Nova is going to want return on their investment (in Harmonix). While Dance Central is no doubt helping here, I don't doubt that they'll want some sort of return from their other franchise. The low investment needed (see point 1; they can re-use a lot of what they worked on for RB3 and add a few new stages and stuff) means less of a need for stellar sales figures.
Their announcement was obviously geared towards the MOAR METAL crowd, and on that front, it probably satisfied people that are neither you nor me. I didn't care for most of those names (I only care about the Soundgarden tracks we already had and lost; Poison is fine for a couple of songs).
This week's announcement, on the other hand, still contains tons of popular songs that I know I want. Live? Tubthumping? Take On Me? Those are up my alley.
Plus, you can't just ignore the stuff that's been released over the last month. Between classic bands (Ozzy, Foreigner, Loverboy, Stone Temple Pilots) and more modern ones (Bruno Mars, Lady Gaga), they're still bagging a lot of hits old and new, without MTV's influence.
Don't forget: in this very thread are people talking about how their PRE-ORDERS never got filled, which means that Fender didn't even manage to make enough guitars to fill the demand that existed before the thing ever existed. We don't know WHY Fender bailed; we just know that it DID. Maybe--and this is just hope; there's nothing suggesting this at all--they're working on a hardware revision. Maybe something happened with their electronic parts suppliers. Who knows?
Besides, the Mustang is still available if you want to play Pro Guitar.
I still see nothing suggesting that we only have half a year of DLC left. Besides which, even when official channels are exhausted, RBN still exists.