I've remarked before, if every item of household furniture was called a chair, we shouldn't know where to park our arses.

What a vapid analogy, Michael. Furniture is furniture. We have a diversity of types - tables, chairs, sofas, coffee tables, bookshelves. There's nothing inherently (or structurally) different here - they're just different in terms of style determined by their (complementary) domestic function. And within that there are lots of different places to park our arses - stools, settles, thrones, crackets, shooting sticks, misericords... We buy all ours from IKEA in flat packs (shooting and misericords notwithstanding) & I think our coffee table matches our bookshelves - otherwise I don't think there's a case to made that one item of furniture is so very different from another. I often sit on my table - and both of our Poang IKEA chairs are more often used for additional shelf-space than for the parking of arses.

Folk is not 'just a style': it is a category, subsumed within the broader category of music~~

Folk is a myth - it is an illusion conceived of at several removes from the thing it attempts to prescribe. If you accept the reality of that prescription (i.e. the orthodox reading of the 1954 Definition) then for sure, it becomes a category. Folk is not just a style of music, it's a matter of faith. You might say Butter & Cheese & All is a Folk Song for very different reasons than I say it's a Folk Song. I say it's a Folk Song because of its style, otherwise its manifest mastery is due to the same unique creative idiomatic musical genius we find in Bach, Gershwin, Coltrane or Dr Dre.   

That is just nonsense ~~ as you know yourself when you're not playing devil's advocate.

The very nature of Music is defined by Traditional and Ethnic / Cultural context. If this isn't the case, oh wise one, then feel free to tell me one that isn't - and tell me why it isn't. Or even one that can't be slotted into the 1954 Definition if it comes to that.