"If everyone knew the song what would the pro do."

Just how stupid are you?? Do you think we listen to a song once and that's it?? If a professional musician has a song, written or traditional, that is loved by the audience as sung by him/her, s/he will perform that song over and over and over and over again. And the audience will be happy. And the performer, a professional, will perform that song again even though s/he'd rather perform something else.