I don't buy that reasoning for a second. NFL Coaches, by and large, are some of the most cowardly bastards on the planet. They HATE to make changes and they HATE to take risks. Why do you think so many of them punt on 4th and 3 even when playing from behind?
Right now McDermott is like "well, if we play Tyrod and lose too much and miss the playoffs, my exposure to criticism is minimized because I stuck with my starting QB. Tyrod will get the bulk of the blame, we can cast him off in the off season, and try again with someone new in 2018". If he makes the move to bench Tyrod for Peterman now and it doesn't work, he'll get ALL the blame. Tyrod will look like a victim of a bad coaching decision and Peterman will look like a victim of being thrown to the wolves while he was still too raw. It would truly take a coach with balls to make the decision to bench Tyrod at this point in the season.
Seriously though, does anyone think Belichick would still be playing Tyrod? Me either.