Mitc is supposedly trying to be traded to New York, which has worse assets than SA has. The only reason to move him there over SA is if the Spurs aren't willing to acquire him anyway. I'm not even in love with Mitc , but I don't think he's out of the Spurs' realistic market. Irving and Beal are free agents and don't need to be traded.
That still leaves Sexton. Sexton makes a ton of sense as a relatively high-volume efficient scorer who can handle the ball while not needing to dominate it. The Cavs might not want to let him go, but they were willing to last summer and at least will likely be willing to play ball on an S&T as opposed to matching a max offer sheet. He's a solid plan C to the big-name guys and Lavine.