Nah, the trump card is Duncan saying: "Shortchange me and I'll go win a couple championships with the Miami Heat while being a hop, skip and jump away from St. Croix. Meanwhile, Spurs fans will boycott the team like after George Gervin left when they hear how little you offered me to stay despite all those championship trophies I brought to San Antonio."
That, my friend, is game, set and match if Duncan wants to actually negotiate. With that leverage, Duncan basically can get whatever he wants, tbh. If Duncan leaves in a messy way, the franchise value depreciation alone would be a lot more than whatever money the Spurs "save" by letting Duncan walk.