A non-argument, I fear, Sweeney; since in fact, Child used the word 'Popular' in his title as his synonym for 'folk' - in much the same sense that Brand had used 'Popular Antiquities' to mean what we would call 'folk customs' a century before — see my entry on Folklore in the Continuum Encyclopedia Of British Literature {NY 2003} where I open my entry by dealing with these differing nomenclatures for what we generally call 'Folk'. When Child called his collection 'The English & Scottish Popular Ballads', he certainly did not mean what would nowadays be called 'popular' [or 'pop'] songs.