I don't know how some cannot see the improvement in his defending. A couple of seasons ago, he was very very dodgy from a purely defensive POV. Now he is as solid, if not more so than any of our full backs. I don't know the exact stats, but I would be amazed if he has made more mistakes leading to opposition goals then either Walker or Cancelo and currently he is so superior to Mendy, its not even worth bringing him into the discussion. As for replacing with Ake, no thanks.
My only wish for Zin, is that he gets his head up a little quicker when he gets the ball. He has so many opportunities to play the ball in behind, but he invariably holds on just that split-second too long and the opportunity is gone.