Handel's Messiah . . . ? Sorry, I'm afraid I'm from the school that says it can borrow from, or grow out of, folk tradition, but that doesn't make it folk.
At the risk of being redundant . . . I guess that I see a lot of genres of music being played by/for a lot of different kind of folk (sure, rock could fit this when it's a garage band writing its own material, and there's a very good modern jazz band on campus that writes a lot of its own songs), so I think that definition doesn't really mean anything. I'd like to see some stylistic restrictions in there someplace. Folk may be a root for a lot of stuff but I don't see that it has to hold onto its "children" as sub-genres of itself.
I have yet to come up with a defintion that really satisfies me--"I know it when I hear it" is pretty nebulous. But I don't mind ousting music from the category when I must admit it's overstepped the gray area.