There must be something strange in the air for me to agree with you, but on this one I do!
Delph has got all of the physical tools to be a great left back. Naturally left footed, perfect size and build, fast, athletic, good in the tackle, decent passer of the ball, decent crosser of the ball.
He's looked dreadful at left wing, but I get the impression he's just been picked there and told to get on with it, no coaching or instruction whatsoever and he's looked like a fish out of water.
If Pep wanted to turn him in to a left back he could spend the time coaching and developing him, moulding him in to a Guardiola left back.
The role Guardiola expect from his full backs is more in line with a midfielder than a defender anyway. It's a very specific role that you don't see other manager's playing.
Even if we bought a specialist left back, they would still have to be coached and alter their game to suit Guardiola's system. No reason Delph couldn't be