Anyway, as to try to save this thread, even though it's probably too late now, I'm gonna go back on track and explain why it is a fitting song:
The parallel between the phrase "the day the music died" and Rock Band DLC ending is unquestionable. It's also a very well-known song that almost everyone and their grandma knows. It's depressing, sure, but it fits perfectly because of it's overall theme of looking back at Rock 'n Roll.
Sure, I would have personally went for something more "epic" like Octavarium by Dream Theater, but this is something a lot more people want, and I think that it's a fitting song to close out a game like this, much like how it's usually the last song at karaoke night when it isn't Freebird.