For the Spurs a move like this makes perfect sense. With a generational talent like Wemby, you want to do everything possible to help him. Giving him a former all star guard is a great start. Worse case scenario for the Spurs Garland ends up being an expensive guy to throw lobs, but I do believe Garland still has a higher ceiling than any guards the Spurs could take with the 8th pick, and they still have the 4th pick if they want to double down.
For the Cavs it’s as simple as giving Don more touches. Keldon can fit well at the starting 2 or 3, Collin’s is a great backup big especially if Allen gets traded, and Jones is a serviceable backup PG. throw in the 8th pick to trade someone like Matas or Ron, along with a potential high upside pick next year and the Cavs have a good start to a potential re tool