Yetties & Spinners did sing mainly folksongs, in fact. Not everyone liked all their arrangements -- I once had the privilege of supporting Hughie Jones at a gig; so I greatly welcomed the opportunity of telling him how much I loathed & despised what they had done to that beautiful song Pleasant&Delightful, with all their silly noises & gestures -- but their repertoire was nearly all folk at that.

Yes,"common usage" is indeed a very well-chosen name for the phenomenon you rubricate.

    ······ Common as piggipooze; vulgar as all-get-out······

You can urge universal ignorance till blue in face. So what of it...

You go on calling them folksongs if it gives you any satisfaction. & I shall call my tallest bookcase a chair. But I won't try sitting on it.

≈M≈