"...when 'I Don't Like Mondays' is given a Roud number (given Roud's re-definition - why not?)"

For one thing, Jim, Roud regards folk song as a historical phenomenon, and makes it clear that the folk revival is outside his field of interest.

For another, even if you accept that the revival repertoire constitutes a tradition of itself, 'I Don't Like Mondays' would be a very weak candidate for canonization. I've never heard it sung from the floor in all the years I've been going to clubs and song sessions, and if the much-respected but famously eclectic Dave Burland hadn't started performing it several decades ago, it would never have come up in these discussions.