Your definition of world class is harsh tho, in my mind one of the best in the world is a top ten kind of deal. Position specific
And I'd say he does have the potential to be a top ten left back. Its debatable sure. Whether I'm biased I don't know, maybe just more interested cos he's Scottish.
He's not Marcelo or Jordi Alba sure, but every next level he reaches he adapts and thrives in. So it's hard to say how far he can reach.