What I alway say is(along with Bert Lloyd) is that if someone wants to include modern written songs in the category of folksongs, then they are duty bound to think up a good word to describe the things that used to be called folk songs.

Then that word would be Traditional song, which the evidence would suggest is in no way synonymous with Folk song which research has shown can include any song of any genre sung in a designated folk context by designated folk singers, including, of course, Traditional song. Furthermore, any Traditional song can be sung in any other musical context - rock, classical, jazz, experimental, pop &c. - without any loss to its essential integrity. Again, this isn't idle theorising rather an observance of the available evidence.