The problem for Pep is he hasn't had the players at City to implement it himself. We've never had natural options at both left and right wing-back at the same time, and never had three central defenders who are good enough at bringing the ball out until this past season
That's true, I would have thought when we bought Cancelo, than he and Mendy would work, but Mendy's constant injuries meant we rarely seen this, plus his down form.
That being said, Pep has cleverly himself transitioned to a back 3 with Cancelo/Zinchenko playing inverted going from 4141 to 3241, and using wingers instead of wingbacks.
The problem we have had with this is that it isn't "natural", because then if we get countered, Cancelo has to go from inside to outside to get back to position. A default 3 atb is more seemless when defending, although there is ALOT of merit to Pep's approach in using "wingers" instead of "wingbacks" that I like.
Perhaps a more natural way to preserve "wingers" over "wingbacks" is for Pep to play Rodri as a CB with Dias, and Stones and Laporte as fullbacks, in a 451. Then when we pin the opposition back, Rodri pushes forward to join the midfield, to make a 3151, seems more logical to have Rodri slot back, then Cancelo slot back when comparing.