Your theory, BobL, is borne out by the fact that if you look in books of folk songs, containing mostly old songs but some more modern ones, you'll see that most of those written and published in, say, the later 20th century, have sunk without trace, whereas many of the older traditional songs are still being sung. There are of course some modern gems that have passed into circulation and are cherished as much as the older songs, but most of the modern ones don't last - just as most of the old ones probably didn't either.

Marje