This isn't a question looking for an answer. It's an excuse for a conversation. Accept it as that.

The key thing for me is that a folk song is a song that was either never part of the music industry, or has escaped from it, and gone feral. People learn it from each other, and the fact that it has been recorded by a particular person on a particular occasion recedes into the background.

Some songs start off as folksongs and get temporarily captured and tamed or put in a cage. But they can always escape, and so can the ones that were born in captivity. Some of them die in the wild of course, because they haven't got the qualities that enable them to survive. (I'm getying back to the horses...)

Which gives rise to another way of putting the original question: "What are the qualities that enable a song to become a folksong?"