Even if he has made up his mind that he's leaving he'll want to play at his best to get the attention of the clubs he wants to sign for. A lot of players play at their best in the last year of their contracts. Also it's not in Sanchez's character to not give it his all on the pitch, and from his comments about the decision being down to the club it doesn't look like throw his toys out of the pram if we make him see his contract out.
But I am split over whether we should sell him or not. I can see the argument for keeping him but how painful will it be if we still don't win anything and let him walk to City for free in 12 months? If I had more faith in Wenger delivering I would probably lean towards keeping him but I'm not feeling very positive, with or without Alexis.