It's interesting that for each criteria, we can always find an exception.

Simplicity?   Well how do you explain "Raindrops Keep Falling On My Head?"

A good melody?   Would You are My Sunshine apply?   Think of other songs that have a dreadful melody but everybody still sings.   Can't off the top of my head......but I do remember remarking on that with a few songs.

Timeless lyrics?   Well, I guess that's tautological.....if the lyric lasts forever, than I guess it's timeless. But what really makes them timeless? For example "I ride an Old Paint".....very few people even know what most of the words mean, but we sing them.   And I'm sure there's lots of better examples of that.

I'm sure there must have been some actual research done on the factors of a long lasting song.   I'm wondering if anything definite has been found, or whether it really is a matter of chance (or some version of 'payola'?).