> but folk(2) is just "whatever people are calling folk at any given time".

Unfortunately, any word can be defined that way, which tells us very little. ("A 'keyboard' means whatever people are calling a 'keyboard' at any given time.")

Yet we know pretty much what people mean when they talk about a keyboard, love, birth, death, even infinity - even if the details are fuzzy around the edges.

Nowadays "folk" seems fuzzy all the way through.