Folk is the simple fact that people do such things, seasonally, ceremonially, festively, ritually, by way of observance, remembrance, celebration, recreation, entertainment and devotion.

Folk as folk performance - that might work. But it would mean that lots of things that most people call folk aren't, and lots of things are folk that most people - including the people involved - don't call folk. (The Dylan night I went to last week was almost entirely folk by this definition.)

I think something definitive of 'folk' gets lost if the folk artform itself isn't continually evolving and changing - which is almost bound to be the case if you're using traditional material, but not if you're trying to sing the words Dylan wrote the way Dylan sang them.