He will never, ever, ever, ever leave the Spurs. I can't think of a player who is more singularly associated with a particular organization. Even other star players who've only played for one organization - Kobe's a Laker, but he flirted with free agency and they've flirted with trading him. Pierce will retire as one of the all-time Celtics greats, but you know that Danny at least was open to trading him.
There's never been a single moment since Duncan was drafted where you ever thought he was going anywhere else, either of his own volition or the team's. He IS the Spurs franchise.