Really, it's the next logical jump. I, and I'm sure everybody else, has thought about how they would do it. The South Park episode really did prove a good point: the translation just wasn't there yet.
I'm kinda surprised that Harmonix hasn't been the first one to do it, though. It's a bold leap, but it must be made. If you have four-year-olds doing speed runs on Expert, you just need to give him a real guitar and see if he can do it with a real instrument. Pull that off, and you might actually turn him into an excellent shredder of guitars. That's actually worth something in life.
Rosalina: But you didn't.
Robert: But I DON'T.
Rosalina: You sure that's right?
Robert: I was going to HAVE told you they'd come?
Rosalina: No.
Robert: The subjunctive?
Rosalina: That's not the subjunctive.
Robert: I don't think the syntax has been invented yet.
Rosalina: It would have had to have had been.
Robert: Had to have...had...been? That can't be right.