Just where do we draw the line......a song can be a hit for a lot of reasons not just the lyrics and melody......It could have great production that tuns a mediocre song into a hit.....It could have a great arrangement...It could have a great instrumental solo.....All these things and more contribute to the song being a hit. Should everybody who contributed any part to the writing, performing, recording, mixing, and mastering also be cited as cowriters?
Example the James Bond famous guitar riff was an ad lib by a guitarists paid basic rates on a work for hire. He made nothng else out of it. Worse still most people do not know who he was.