Maybe the kid pans out, he has athleticism through the roof. I dont really care if he does or doesnt. What he has shown is that his bb iq is low. He makes bad decisions and doesnt play defense despite being athletic and good size for his position, which means he is trying or is really dumb. Bot of those are a problem.
What Exum has shown is that he is elite at defense at his position. He has shown potential to shoot, drive and pass the ball well and is widely praised as very high bb iq, high effort and a hard worker. Those are traits that are valuable at the PG position. PG demands a high IQ since the ball is in your hands so much. Zach has only showed flashes of offensive talent, the only thing he is elite at is jumping. That skill while nice is not all that important from your PG.
In short maybe he ends up being better than Exum, it has a possibility of happening. But right now all signs point to Exum being better and it would be foolish/dumb to make that trade with the information that we have right now. Fortunately the Jazz know and like what they have in Exum. The injury was a real bummer in the short term for this team and Exum. But that does not change his career trajectory and he will come back with more athletic next time you see him in a game than before the injury.