I bolded part of your post. I honestly don't know what to tell you if you don't understand how having a quick hook on your quarterback can hurt their development. You have two seconds from the snap to drop back, survey the field, and make not only a solid decision, but with good technique and accuracy. There is no room for waning confidence. You saw that this weekend. Murph was nails early on but a couple things went wrong and next thing you know he is holding the ball too long and throwing the ball late. You pull him at that point and he is questioning whether he has the ability to play at this level. Then what if Arch is awful and you have to go back to Murphy? You've lost him at that point.
Right now Murphy is pressing, thinking he has to make every play. Why is he doing that? Because he knows who is behind him and feels like he has to play remarkable in order to stay ahead of him. How do you fix that issue? You show him that its ok to take whats there, and its ok to not make every play. You stick with him, coach him up, and get him to settle down. If you bench him you further validate what he is already thinking.
Listen, Murphy is a couple tackled WRs away from having over 300 yrds and two more TDs. Possibly 3 if Cook doesn't let himself get muscled. You get the kid to simply understand to throw the ball away or take a sack when there is nothing there and he is a solid QB. We don't run for 200+ if he is not tearing Kstate a new asshole anytime they showed man coverage.