I think that the joke doesn't finish is a happy accident that plays like a stroke of artistry. One the things everyone likes about the Breakfast Club is not knowing what these character's lives will be like after this amazing day. So much can be speculated on about what happens after the movie ends, it feels like the story itself doesn't end with the credits (like that article you posted about it a few weeks back pointed out). A joke who's ending the audience never gets to hear plays into that thematically.