It's not just collectors who write things down. People who are literate write things down in case they forget them. They also write things down so that can learn them. Or because the person who knows won't be with us for ever - for example the song that granny sings at Christmas.

A family member writing down the songs that granny sings may not need ot want to distinguish between an old Wassail song or "I'm for ever blowing bubbles"

I am suggesting that the moment there are literate people around a pure 'oral tradition' is unlikely to exist.