It would be fascinating to have heard the negotiation. As others have noted, the risk for the Warriors is low. Cousins obviously wants a ring, but he must know he will have to compromise as a player far more than he ever has before or else this won't work.
He will be slowly working himself back into a role for a few months, and when he first starts playing, he will be a significant liability until he can play decent defense. I'm not sure I see him handling a slow, grinding, often frustrating comeback process combined with unprecedented deference to his star teammates, but if he can handle it, yikes!