If a definition of folk has been researched for over a century, it wasn't well documented or promoted.

Millions of people on the planet have in their heads what they consider to be folk. That's a million different subjective ideas.

Ditto pop. (and then some)

I can't tell Jim Carroll what is folk any more than he can keep trying to say I and others are wrong.

If baroque, chamber, orchestral, opera, piano, string quartet, new age, madrigal, blah blah can all fit into classical without anyone cocking their rifles, then traditional (whatever that means) can be a form of folk, not THE form.

It isn't hard. It just means getting out more and starting to enjoy music.