Getting back to the thread, if you call yourself a 'trad' singer or a singer of traditional songs, why on earth would you go anywhere else than the 'source singer(s) for your model? Even if you heard it first from a professional, surely you'd want to know where he or she got it?
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Professional folk performers will 'arrange' their own way of delivering a song/tune but as Ray P says, in normal times they will be paid for the performance, but surely claiming 'trad.arr' this kind of joint venture is irrelevant, and certainly should not imply any kind of partial ownership!
There might be a case of some kind for acts like Thin Lizzy, who almost completely changed the nature of 'Kilgary Mountain' but that may be for the courts to decide on a one-off basis?

As for it being necessary to prevent the songs dying out, the vast amount of performance of traditional songs is carried on 99.9pc by people who never earn a penny from it- so no danger there.
About Peter Kennedy, I'm well aware of his history, but was just wondering what, specifically, had ruffled the Sandman's' feathers. It has probably been explained elsewhere & I accept that now is not the time to pursue this.