It would be better for both clubs to sort it out. Arsenal must know now that he is not going to sign an extended contract. If they are going to sell him, it would probably make sense to just get it done and move on. Dragging it out to make life awkward for us, also makes lie awkward for them. Presumably they want to spend in the transfer market and as you say get their fans heads around it all - and players. I doubt they'd want the opening weeks of the season dominated by is he staying or is he goingYou have to be optimistic Sanchez is on his way when the media start reporting massive wage demands from him and his agent (400k a week +) Making him out to be a mercenary and only after money. IMO Arsenal have briefed the press about this to demonise him and get fans to turn against him and make his departure more palatable. I would also think it would suit them getting the deal done now so fans accept it get used to it before the new season starts, If they leave it till after the season starts no doubt the fans would kick right off and be on Wengers back from the off.
It does seem a bit more likely at the moment but every Summer we seem to have a transfer saga and they are full of ups and downs.