Mayhap there's a Darwinian principle at work, which has kept all the good songs alive to the degree it is sensed they do something for people's survival potential.
A good "meme" has a life of its own, and a good folk song is as lively a meme as ever infected a mind -- witness the way they creep into your head and won't let go.
Memes have a way of surviving that is independent of financial factors, for the most part, but not independent of people's considerations about what they want in the future, which is Darwin's point about surviving organisms -- they're better equipped for moving forward in time.

A