"Count yourself lucky you have someone who can play it (Candy Man)in your gang. However if he can play that all right, he should be able to play pretty much anything, in the three chord lexicon of a million folk songs"

It ain't necessarily so.... there are those who practice diligently a very limited number of songs, often using the "play by numbers/master the tab" approach to rise above the usual level, but who certainly avoid a more casual broad-based "wing-it" approach, or shy away from innovation.

Singer-songwriters who hit the road to promote their songs/style also (by choice or necessity) stick in a narrow groove.

Each to their own, but I agree with the original point made; though 'churning out stuff he did from 30 years ago' is not limited to floor singers.