KG has made $270 million in his career. I seriously doubt he stays because of the money. If he is ready to walk, regardless of his skill level or money left on his contract or money he can still make after his current contract expires, he will walk away.
The loss might bother him because he sees that he can't bring it consistently every night in the playoffs against younger, stronger players and he knows in order to succeed in the playoffs you have to bring it every night. He could be coming to the realization that his time in the spotlight is over and will decide to leave.
If he stays or retires, neither would surprise me.