Traditional to me means so long ago that most people or indeed no-one at all knows who wrote it. Even time served songs by McColl or Cyril Tawney, written in the style of a folk song, passing through variation and selection etc. to become what many would reasonably call a Folk Song are not Traditional.
The origin of a song can no longer disappear into the mists of time. Today's media and access to information means that in centuries to come our descendants will still know who wrote Shoals of Herring so it will never be traditional.
Equally so a pop song.