I think the problem here is that we keep using the word 'definition' for something which can't be defined.

In my opinion, the only sort of 'folk' which is open to definition in objective terms is traditional music, where the 1954 definition or something like it sets out the characteristics of what makes a traditional folk song, and distinguishes it from a popular song that is widely known (we can argue what those characteristics might be, and whether the 1954 version is correct or complete, but my point is it is capable of being defined).

The other sort of 'folk' cannot be defined. It depends on the expectations of its audience and what they are willing to accept, and to an extent this will depend on context - Cambridge Folk Festival's audience's idea is rather broader than mine (let alone Jim's!). You can describe certain characteristics which may make it more or less likely that a song will fall within these expectations and achieve broad acceptance, but these can only be pointers. You can't 'define' this meaning of 'folk', it is simply too vague.

I suppose that is Jim's point, but being vague doesn't mean that it doesn't work in practice. Those who go to Cambridge Folk Festival know what to expect, and those who avoid it do so for the same reason.