Not all of Chicago's music is Classic Rock. "If You Leave Me Now" is a pop song. So is "Hard to Say I'm Sorry." Billy Joel spans a bunch of labels, but I can't think of any of his that would be Classic Rock. He barely has any rock at all. I mean, we might as well include Jim Croce and James Taylor in the category. As someone else said in this thread, it has to be guitar-driven to be Classic Rock.