"One swallow, as they say..... "

Well, by 1973 we were well into Autumn, if not Winter, as far as folk song (in the old sense) is concerned. On the other hand, hundreds of independent versions of 'Lord Bateman' were still doing the rounds in various parts of the English-speaking world in the early part of the 20th century, which isn't bad for a song that was at least 150 years old by 1900.

The point I'm trying to make is that songs of recent composition, whether 'Johnny B. Goode' or 'Bluebirds', are never likely to 'become folksongs' in the way that 'Lord Bateman' did, because we just don't live in that kind of culture any more. 'Oh Susannah' probably has 'become a folksong' as Uncle Dave O suggested, but then that was written in 1847.