His performance keep dropping since the last season, this season even worse.
It's hard to expect his performance will back in the next season, but the problem is Manchester City currently don't have a deserved player who can replace De Bruyne. Extend his contract for one more year doesn't sound bad.
Manchester CIty can use one year to find better player, whether it's Marmoush or sign a new top attacking midfielder.
Players like KDB are not given an extended contract only because of their performance but for experience and past contribution to the club. His recurring injuries might be another reason why Manchester City might want to let him go. It could also be that he wants to leave to gain experience from another league..
Ordinarily, Pep Guardiola wouldn't allow a player like Kevin de Bruyne to leave despite the reoccurring injury problems, I think it is the player himself that is insisting on leaving the team, I think the player believes his time at the Etihad is over and perhaps it is time for him to leave the club, his wealth of experience has been duly utilized at the club and the evidence thereof are too numerous to mention because Kevin De Bruyne is one of the most fulfilled player in the Manchester city squad at the moment hence he has nothing to lose if he leaves now and for me, I think his decision should be respected.