There are so many examples of music that are regarded as "folk" today that it makes this discussion so general as to not be understood by anyone. The so-called "folk canon" is being heralded by self-styled authorities who operate from misconceptions. The basis for folk music has to be cultural and traditional, that is material that is handed down through generations and reflects the environment from which they came. Lenon and McCartney do not fall within this classification. Nor does Punk Rock. Nor at the present time does Hip Hop although there are some aspects of this music that hark back to earlier roots.
I think that many who would advance their theories of what folk music is do it to furthur their own professional agendas. In other words, if the music sells somehow we ought to be able to label it folk, a spurious conclusion.