A couple of things re Stones, other than him not being that old. Alongside Laporte, he had a 30+ game run where they hardly conceded a goal, never mind lost a game. Obviously a lot of that is down to Laporte, who I see as a more consistent, left footed version of him without the mistakes. However you don't post those sort of stats unless you are very good.
I'd also add, at times Stones has shown fantastic 1 on 1 defensive ability here. I remember him going to Old Trafford and dominating van Persie. Pre season match as well he kept Zlatan quiet (just before he left).
I think he's lost site of a lot of this. I think he tries hard to try and prove he's world class rather than just do the basics. He makes the odd mistake, but he's 6 ft 3, quick, reads the game well. There's probably an issue of focus. As always if the deal was right he would be a good signing.