Turning 29 soon, his game has never been about pace... if he can stay mostly fit we've got at least 5 more seasons out of John. By the time those are up he could genuinely be recognised as the best English defender of all time. He's phenomenal.
I have nothing against Laporte, but for me, the only good argument to start him over Stones is that Pep likes a left-footed centre-back. Laporte's world class, Stones and Dias are probably the two best in the world at the moment.
Also, not sure how relevant this is, but if City were to win the Champions League 1-0 (and I have thought hard about this) I would want Stones to score the winner more than anyone else.