Well Don to be truthful I wasn't quite sure why you had started this thread, but I suppose some of the responses have provided the answer. Certainly in folk music there is as wide a range of styles and abilities as in any other field of human endeavour, and as wide a range of commercial success and popular acclaim. One thing I think is certain, there is no correlation between commercial success and ability, and unfortunately there does seem to be an inverse relationship between it and approbation by adherents of the genre. However that does not mean that the inverse is true and that anyone who has any commercial success is bad.
I would agree that folk music is constantly developing, and will absorb influences from other fields, but I would suggest that Colin Stewart has his direction inversed, Jazz, Rock and even pop music derives essentially from various folk styles. That is not to say there is not a two way flow, just pointing out the main direction. I suspect that the reason I like the Corries and feel obliged to argue with Colin is that, like Don, it is the words and singing that I appreciate most, some of those on your list Colin may be technically excellent musicians, but I would go a long way not to have to sit through their performance which frankly bore the pants off me.
I would also have to take issue with Colin on the issue of "ripping off" the system. Given your age Colin, you obviously weren't around during the great folk disaster, but in your terms the Corries, Clancys and Dubliners, Seeger, Guthrie, MacColl etc all ripped off the system, and if one wishes to be cynical, then all the people you mention could be accused of continuing to do so by allowing themselves to be marketed under the "Celtic" lablel. Hell, I even remember Dick Gaughan when he was a folk singer, before he started ripping off the system by writing his own stuff.
Each to his own Colin, I'm pleased that you have strong views and are prepared to argue them, I don't particularly want to convert you, but just be aware that your arguments can as easily be applied to those you laud.