KG has been such an amazing player his whole career, I guess I don't see him sticking around past where he feels like he can make a major contribution to the team. The dude is a proud man. If he feels he is slipping and he can't be the player he wants to be and can't feel as healthy as he wants to feel I wouldn't be surprised if he retires.
I am not sure the rebuild thing is huge or not. From his interviews, he seems to love working with the new / young guys. The guy sounds really loyal to Doc, so he isn't just going to ship out with no reason. Maybe it would be more on his own expectations of his level of play and health.
I would love it if he played out his whole contract, but that is because I love KG.