There seems to be an Anglocentric view when it come to defining folk songs.

There are folk songs all over the world and I believe "folk song" should always be plural.

My opinion is that folk songs are generally accessible to those who want to learn them.
Not like an opera aria or concert art songs. Their durability has to do with being accessible and easy enough to learn to be handed down from one generation to another.

You have to train hard to sing arias, motets, art songs etc. Even jazz songs require some amount of musical training although Ella Fitzgerald or Billie Holiday seems to have by-passed that.

I place blues into the category of folk songs. Traditional Appalachian songs and dance tunes done by bluegrass musicians are folk songs. There are European, Asian, African, Indian etc. folk songs. Folk songs have a direct relationship to a sub-culture. They often define it.

I submit that folk songs are communal in their evolution.

You don't have to be a traditional emissary from any particular culture to sing them and enjoy them.

I do believe though that you have to put on a different set of ears to appreciate traditional singers..It's difficult today with people having fast paces in their lives to quietly sit and listen to a twenty stanza ballad or a low decibel level a-capella singer. I personally find this satisfying. Untrained outdoor voices have an appeal for me.

I believe that people became interested in this form of expression when they were exposed to it without preconceived musical prejudices. Some old guy without teeth sitting on a log or farm fence post hollering out a traditional song can be an edifying experience or a blues bottle neck National Guitar player from a poor black community can be entrancing. You have to leave a lot of learned musical responses behind.

I took a commercial music course at Georgia State University with a lot of would-be rock producers, musicians, future commercial music moguls, mainly young people, and I brought in a recording of Almeda Riddle. They hooted, catcalled and rejected it. Talk about musical intolerance, there it was. And they would decry any attempt by anyone
to put down their rock music. They needed to put on a different set of ears. Commercial musicians can be big snobs when it comes to their own music.

I don't think it matters whether we define it or not because it will always be there in some form or other....a lullaby, a skip rope rhyme, a dirty song or a schoolyard chant, in a barracks or in a prison, on a picket line, or someone just singing to themselves a song that their grandfather, uncle, aunt, mama or daddy taught them from the back woods, Old Country, from an old hobo, or when they were in the Merchant Marines.....they come from unlikely places. All over the world.