"Well, I've heard heavy metal songs re-imagined as folk songs, "
I'll take it from your evasively obscure answer that I didn't get it wrong - so you have your response.
"And what vested interest might that be?"
Fitting your particularly square peg into a round hole - no symbolism intended!
"You mean they don't?"
Not if they try to pass off your rag-bag as folk, they don't.
T.J.
"I disagree that we have no more to discover."
Don't think anybody is trying to say that; rather, that traditional song has ceased to exist as a living form, just as nobody wrote Shakespeare plays after he died - nothing to say that they can't continue to be performed and enjoyed.
There is loads more to discover - we know next to nothing of what the traditional singers thought of their art - because, in the main they were never considered worth asking - hence the mess.
"Seems to me that nobody defined traditional song,"
Yes they have - it just doesn't suit some people.
"Does it need to be sung in some "traditional" manner...."
No - style or setting has nothing to do with it's 'folkness'.
It needs to have undergone a process which makes it folk - been through that thousands of times.
"As to the other question of whether COLLECTION of traditional song is finished. I doubt it."
Would love to agree, but not in our experience. Doesn't mean we've got nothing new to hand. Following the Greig Duncan Folk Song Collection relatively recently publised we have yet to see the J.M. Carpenter collection, arguably the largest single collection of traditional ballads ever gathered.
Jim Carroll