In the "good old days" you could at least count on a verse, chorus, different verse, chorus, etc. structure. Then at some point they were doing "chorus, chorus, chorus, bridge, chorus...". Soon we'll be lucky to get more than a repeated sentence fragment.
Case in point: https://www.google.ca/search?q=turn+down+for+what+lyrics
Or this gem: https://www.google.ca/search?q=watch+me+whip+lyrics
A better example of a pop song might be Twist and Shout; not much variety there either.
Latin church hymns, for example, largely use the same corpus of Latin for lyrics, but no one is about to accuse Mozart or Beethoven of a crisis of originality in lyrics.
Case in point: https://www.cpdl.org/wiki/images/3/32/Mozart_427_Great_Mass_...
That's not a good metric.
If I looked for a good metric, I would probably take min(word per meaning and touch), but that would raise a question on what ‘meaning and touch’ is. Nonetheless, good lyrics always seem to be un/under-spoken or sharp cut.