"I was meaning that there would be 18th century Walters"
I'm still not sure what you mean
People didn't select folksongs - they chose songs to sing which became 'folk' by their absorption - the definition was done by outsiders, the acceptance was the beginning of the making of a song which then evolved into a folksong
The process whereby that happens is and I believe will remain a mystery - we didn't spend enough with the singers to find out why they identified with and passed on their songs - we can only guess that

I never get tired o quoting the Jean Richie statement from when she was collecting songs in Ireland, though I'm sure ther are many who get tired of my doing so

“I used the song Barbara Allen as a collecting tool because everybody knew it.
When I would ask people to sing me some of their old songs they would sometimes sing ‘Does Your Mother Come from Ireland?’ or something about shamrocks.
But if I asked if they knew Barbara Allen, immediately they knew exactly what kind of song I was talking about and they would bring out beautiful old things that matched mine, and were variants of the songs I knew in Kentucky. It was like coming home.”

Why on earth should Irish country-people identify with a (possibly English) ballad that predates The Great Fire of London ?
But they did

I've just heard that an old singer we recorded (now aged 98) is till very much with us and anxious for company
Joe Coneely was extremely vocal as to why he selected and rejected his songs - he dad six Child ballads in his repertoire and around ten other excellent songs - he probably knows many more but chooses not so sing them   
Perhaps we'll get a chance to ask him why
Jim