OK, Jon, I see your point. But can you compare that ? Classical componists write music for a full orchestra, while folk is for a few instruments, or even a singing voice only. Folk is some kind of the people´s singing voice. What I try to say: Folk deals with the people´s problems or at least things that are important to them. That´s the root for folk lyrics to grow, even for choice of instruments - remember lilting in Irish musical history. On the other hand orchestral music from the classic period was to satisfy aristocrats.
So, in my opinion, one should rather compare folk and pop to draw the line that limits the boundary of folk music. The reason is, because also pop has it´s roots in all the stuff that people are engaged with (additionally the focus point of making money...) so the roots (the source for lyrics and the tune itself) of pop and folk are the same. Just the intentions and the realisations are different.