Fox’s contract makes him untradable.
He was bad in the Finals, but they probably don't get past the Thunder without him. He came up big in game 7.
He signed under Bird Rights, so the acquiring team could exceed the cap and bring him in... He was an All-Star this year and is only 28, but he's not much of a 3 point threat. When a guy is as reliant his next level athleticism as Fox, you'd definitely worry about the last couple years of the contract, especially when he's making 50+ million.
It seems like he's an older, and not quite as good version of Stephon Castle. Clearly, the Spurs would benefit from adding someone who can really shoot it. They have plenty of guys who are capable of hitting a 3, but no one who is great.
One interesting proposal I saw, was Fox and two1sts to the Nets for Porter Jr., who is also up for a significant extension and is also eligible for Bird rights. He would give the Spurs some needed length and scoring punch. MPJ isn't the guy for the Nets to build around, he's better suited as a 2nd fiddle, IMO.