"Jim, definitions change because of popular usage, ask a linguist"
Yes they do
Unfortunately folk has never become popular enough to bring about such changes
The term simply doesn't have popular usage when applied to song and any changes have been brought about by self-interested groups who either don't know or don't care what the term means when applied to song.
As I have constantly said, bring me a new definition and we might be able to discuss it - constant and unqualified repetition of the same points seems to be getting us nowhere, very, very slowly
Jim Carroll