Cavaliers are super expensive, they aren't built for title contention, Harden is most likely picking up that $42m player option, and despite the fact that they don't control their 1st round pick until 2030, trading Mitchell is probably their best move.
Mobley can do more with the ball, but he's never going to reach his full potential playing with two lead-ball handling guards. Get as much as you possibly can for Mitchell. You're probably still in playoff contention for the next few years, and you can at least see what you have in Mobley.