Rap can be traced back to the call and response music of field singing and to Jamaican toasting. It also incorporates music. If all that other stuff is folk, then so is rap.

Of course they didn't consider themselves folk singers - the term wasn't even invented when many of these songs were created. And folk as a term was not initially used to recognise "quality and staying power" - it was a largely pejorative term used to distinguish peasant crafts and pastimes from the "high art" created by the educated classes.

I think a lot of your perspectives are rooted in a late-20th century notion of what "folk" means.