If he wants to go, then sell him!
If he thinks that he is irreplaceable, he will find out in short order that he is dead wrong! Pellegrini gave him the freedom to play without much responsibility. To do that, he needs world class players around him and a manager who will allow it. Good luck with that!
I think Yaya is an IMMENSE player, when "he brings his boots, not his deck chair" (not my quote!), but we all know what he is like with a strop on. No problem, the price starts in the high 20s, and will aid us immeasurably with meeting the £49M net spend this summer!
He has already said he wants to play in Paris and PSG are in the same position as us, so good luck with that Yaya, too!
I believe in Manchester City and NO PLAYER is bigger than that. If Yaya doesn't want to be part of the clubs future........wish him good luck, thank him for his service and cash the big fat check.....at least one top quality player and TWO top salaries!!