"but that there was little incentive for them to do so as there were plenty of songs coming out of the towns and coming down from above in the period under discussion."
You miss the point Steve.
People made songs, not because they lacked entertainment, though the 'entertainment' factor was always the main one, but because they felt it necessary to record in some way what they saw and experienced, and how they felt about it; the hardship that was part of their lives acted as an incentive, not a barrier to making songs.
To suggest that people would be happy to 'contract out' the recording of these experiences is as nonsensical as suggesting that sailors didn't bother making songs and spinning yarns because Herman Melville and Richard Henry Dana had done the job for them - this goes for mining, the textile industry, agricultural labour.....
Walter Pardon preserved a small number of locally made songs and squibs on the re-establishment of the Agricultural Workers Union in East Anglia in 1906, he said there were more he never learned - the one that 'made it to the charts' so to speak was 'The Old Man's Advice'.   
Irish, Scots and English Travellers, as well as the large number of Traditional songs they preserved (in their case, without the aid of literacy), hundreds of songs we know about which recorded everyday incidents of their lives - nobody knows how many disappeared as these incidents faded from the communities along with memory of the events.
I worked in Manchester Library in the 60s on their collection of campaigning newspapers, many of which ran a regular song and poetry column - hundreds of songs by textile workers that never survived beyond the events they dealt with, apart from the few that were published by worker poets like Axon. Laycock and Bamford.
This area in the West of Ireland has produced hundreds of local songs, a few of which have survived in living memory, but many more we only know existed because somebody remembered they did - unfortunately not the songs.
This overlooked repertoire is part of our oral history and, I believe, our song tradition, and it is an indication of the fact that working man is a compulsive song-maker.
Our extensive knowledge of that tradition dates back to the end of the 19th century when song collecting began in any sort of organised and extensive way.
It has always been assumed that our oral traditional repertoire was made by the people whose lives it recorded, hence the reference "country songs".
Nobody is denying that the broadsides played a part in circulating a great many of these songs, and even produced some of them, but if you are going to claim that they account for the making of 90% plus of them it is up to you to prove your case beyond the first printed examples - to show that they couldn't have existed beyond the earliest broadside.
You also have to show how a bunch of poor poets (hacks) obtained their skills and insider knowledge to fool people into believing that they were East Anglian, Traveller, Clare..... however the singers regarded and described them, "the genuine article".
You owe it to all of us who care about these songs and believe them to be important to do the job properly and fully - 'earliest' doesn't hack it.
Good luck with that one!
Jim Carroll