"But choral singing a 21st-century phenomenon? "
Choral singing when applied across the board to any type of music (in this case folk) is not only 21st century, but is confined only to the revival as far as I can see (especially as some people here would have it as compulsory unless permission was sought and obtained).
The Copper style is all but unique to one family, and is a good example (imo) of how it doesn't work on all songs It is a family 'tradition' just as singing around the piano can be described as such - do you envisage that approach for, say, classic ballads?
To me, it is as bizarre as Richard's desire to turn us into herd animals, forbidden to create outside the group - Orwellian, to say the least.
Jim Carroll