Once you get out of the academic ivory tower, it all turns out to be quite messy. Many of the revered 'source singers' including Phil Tanner, Harry Cox, Sam Larner and the Coppers are now believed to have acquired some of their songs from printed sources, and had they had access to CDs, they would probably have learnt songs from them too. I've seen articles on collecting songs where it has been stated that part of a singer's repertoire has been rejected by the collector as being learnt 'from the radio', but there seems to be no proof that the version of 'I did it my way' wasn't learnt from a singer in the pub, or 'Lord Bateman' from a Steeleye Span recording on the radio.
In Bob Copper's first book, he recounts how his Uncle Tommy would pick up songs from visitors to his pub - perhaps a drover passing through, there for the one night only. If Tommy couldn't remember all of the song next morning, he couldn't ask the man to fill in the gaps, so he made something up to fit. This aspect of the Coppers' musical tradition seems to have been abandoned. New material has to be brought in to the tradition to keep it healthy - otherwise it's like trying to keep using a river after blocking all the wellsprings.
If the material brought in now is acceptable to those carrying the tradition now, it will flow on - if not, then not. That's the way it has always worked.
Another question - is Eliza Carthy, for example, a source singer because she learnt songs (and singing styles) from her parents? If not, how many generations will it take? Would the answer be different if we were discussing an amateur singer who learnt her repertoire from her parents who were amateur revivalists in the '60s? Real life doesn't fit into neat categories, does it?