The only way the FO ever listens to trade offers is if Duncan wants to be traded or if we would get another total #1 stud in return.
But another team wouldn't want to swap stud for stud like that...they would want Tim next to their guy. And if Tim ever was dissatisfied with the team enough to want to demand a trade, he'd sooner retire.
I believe every GM in the league knows that, unless their name is RC Buford, they will never have Tim Duncan on their team.
They should just hang #21 up there now.