If I remember correctly, Lanning came in at QB, not RB. I believe most of the time DM was still on the field, meaning two power backs in the same set. That made it harder to defend. Does replacing Purdy with Mitchell provide similar strain on the defense, particularly the front 7, to defend in short yardage?
Also, the idea of giving plays to Mitchell when it is not a clutch play, but randomly throughout a game when we have a comfortable lead is not good either. QB and RB's get used to how they hand the ball off. WR get used to how the ball arrives from the QB. One bad read or fumble is a clutch play for the other team and too high of a risk. You don't want Purdy to get hurt, have him hand it off or do a quick route pass play to avoid that.