Going to cost more to replace him than what we could possibly lose if he left 12 months later with only 12 months on his contract.
In any case their is no suggestion that Xhaka would demand a transfer if he was demoted to the bench. Bar the issues he has had with the fans and occasional brain explosion, there is nothing to suggest he isn't professional and wouldn't back himself to keep his spot.
By all accounts he is a model professional in fact. I dont think anyone that has worked with him as had a bad thing to say about him.