P.Kennedy would not record bob blake becauase he suspected he was a revival singer ,fortunately someone else did .

Well he didn't have an obligation to record anybody in particular, did he? If he thought it was a more productive use of his time to look elsewhere, that was his privilege.

What you originally described was a collector going through a source's repertoire and sifting out anything they'd composed themselves. That's not what you say Kennedy did, and it's not what you find in collections like MacColl and Seeger's book on the Stewarts of Blair - they were delighted to find things their sources had created, and to find their take on things that were frankly pop. Even less so for the really serious end, like Vargyas's book on the music of the village of Aj - he wrote down absolutely everything anybody in the village performed during the time he was there. (The book is available in English, it's fascinating).