My point is not that DWill or Mitchell were useless while playing for the Jazz.
It is that for both of them the Jazz was the best situation for their career and their game (good pick and roll PF, good shot blocker to clean up defensive mistakes, 3pt shooters for Dwill; good team with great defensive C, multiple secondary playmakers or 3pt shooters, plenty of shots and touches available for Mitchell). Thats why when/if they leave the Jazz their game was/would be affected and they may not be nearly as effective.
If Mitchell is a bit more self conscious and appreciative of that he would be better off regardless of if he stays or not.