"But since when were the folk interested in defining folk music?"

They never were. And neither am I. And neither should anyone else.

As for a "useful academic classification" - useful to whom? Academics presumably. Anyone else?

Or perhaps you are worried about the "uncountable rabble of ryming ballet-makers and compylers of senseless sonnets"?

That was written as recently as 1586.

"songs with acoustic guitar"? Ah yes, accompanied music. Ever heard of minstrels, with their pipes and harps? They've been around since at least the Norman Conquest, I believe. They were 'instrumental' in spreading many of the ballads throughout the land. Indeed, some may have been composed by them.

Singer-songwriters is the term used today - often with contempt.

There's nothing new under the sun, folks. Step away from the microscope and enjoy the music!