"Good folk". Now that is an interesting concept! Had you said "authentic folk" I would have to agree with you, none of the harmony groups I mentioned sound much like anything I'm aware of from the tradition, even including the Coppers. However the same can be said of most of what is performed in folk clubs, including instrumental accompaniment with guitar, concertina or melodeon, or the rather odd nasal vocalisation affected by some male singers and the breathy head voice of some female singers. The folk revival has developed its own styles. What determines "good folk" in the context of folk clubs is surely whether the audiences enjoy it, and all the groups I mentioned are, or were in their time, very popular with folk club audiences.

I believe the principal reason people become attracted by folk music is the same as any other genre - they like the sound of it. There may be additional reasons of course, but why listen to something if you don't enjoy it? That is the reason I prefer to hear folk songs sung by folk singers (even the nasal/breathy ones) rather than by classical or rock singers - I generally prefer their sound to that of the others.

Let us both acknowledge that our personal experience of folk clubs is necessarily limited and that we cannot be certain that our own experience is entirely representative of the club scene as a whole.    Let us also acknowledge that we have different views on where to draw the line on what is acceptable to hear in a folk club. What I cannot agree with is your assertion that singing songs from the wrong side of that line, wherever it lies, is driving out traditional music and driving away audiences, when folk clubs have always included such material.