There's another issue that should be brought to the surface.

What proportion of our best traditional songs owe their current excellence mainly to the efforts of a few sophisticated rewriters and editors? We don't yet know the answer to that question, but unfortunately it's worth thinking about.

Almost certainly a relatively few writers (several dozen perhaps?) are behind most of the 19th C. broadsides that entered tradition. But those songs have been subject to continual later influences, including the adaptation to (and of) melodies. Modern rewrites tend strongly to be preserved "as is" simply because they're so good already.