Imo, some responses to the question "why our songs last so long?" are the same as some responses to the question "what makes a song good?".

I googled that last question and found what I consider to be an interesting discussion at this non-folk music forum:
http://homerecording.com/bbs/archive/index.php/t-23342.html

Maybe some folks here will also find that discussion interesting-and on topic for this thread.

Here's some excerpts from that forum:

"It takes great metaphors, lyrically. It takes a memorable and catchy chorus. It takes dynamics or some kind of tension and release or contrast. And it takes some element of surprise, wether harmonically, structurally, or stylistically. And I know it must take many more things which I can't quantify."
-Aaron Cheney

**

"A good tune is a good tune....is a good tune."
-RockinRobert

**
"A good song makes people feel something."
-Jvasey

**
"A good song does not require any effort to listen to it."
-CyanJaguar

**
"Personally, I think what makes a good song, is writing material that everyone can relate to...it's being able to write material that has a hook, that everybody can sing along to, and anybody can relate to...that's what I think makes a great song."
-Tim Brown

**
I believe a great song often has an element that contains, and transcends, the precise experience of the songwriter.
-Fab4ever

**
"the greatest songs are those with a universality.
- Fab4ever

**
"If it still makes you cry after the 238th time that you've played it, then it's probably a good song."
-LI Slim

**
...great music stands the test of time...
-Vease

-snip-

etc. etc etc.

In addition to agreeing with those opinions I cited, with regards to folk music [and other genres of music] I think that someone save the music {by putting in print and it being recorded} is a big factor as to why a particular good song lasts so long. Thanks to collectors, we have some but not all of the good songs that were sung centuries ago. Also, I think a song lasts because someone eventually promoted the song. Perhaps it was picked up and sung by someone who became famous or was famous when he or she sung it, and then was associated with that famous person. Or perhaps it was included in some hit musical or show.

Furthermore, imo, a song lasts when that song's words remain understandable [meaning their meaning of its lyrics haven't changed too much, the references mentioned in the song are still known, and their words are still "politically" correct.

That's my story and I'm sticking to it.

****

Off topic, thanks GUEST,Richard and Bert for your comments on my behalf. I very much appreciate them.