In fact I'd hazard that 70-80% of the material I hear in clubs of both trad and mod hue across the land is directly attributable to a 'festival main stage' performer.

I'd be surprised at directly attributable.

btw, a source I didn't see you mention is the events themselves. Probably happens more with tunes than songs but material is learned that way. Then there are the lyric sites which can help fill in the missing words, etc.