You are completly correct. But egos are a crazy thing. Coaches/GMs have done dumber things because of an ego.I would hope that the people running a roster understand and act out that your ego does not matter to on-field results. Playing the hot hand is part of the gig, that's how all other lower drafted and UDFA players break out to have careers. They have to play to show the potential being unlocked. If Max Brosmer is somehow an NFL QB right now, then let him be an NFL QB. As long as you win games that way, the rest of the details don't matter.
My suspicion is the people seeing him in practice everyday know he has deficiencies to be exploited as a regular, green starter in the NFL and they also don't want to give off the mirage of a UDFA winning one surprise game, then turning back into Carson Wentz after that when teams get tape on him and his tendencies.
Once the game was out of hand (half time, honestly), and it was clear Wentz was dragging his left arm, put Brosmer in. Even if he lights it up and then turns into Josh Dobbs next week, at least you will know. But you never find out what he can do if he stands on the sideline.