It's still a silly argument. McCarthy's record speaks for itself. The idea that if you have a great QB any schlub can coach them to a championship is flat-out false. If you think McCarthy has underperformed, your expectations are way too high. Even with a great coach and QB, winning the Super Bowl is really hard! I mean, in the 9 seasons from 2005-2013 the Patriots made the playoffs 8/9 times, went to five conference championship games, and won zero super bowls despite having the best QB/coach tandem in league, while losing five playoff games as a favorite. You need a good deal of luck to win multiple championships.
McCarthy has blown some key games with dumb in-game tactical decisions (the 2014 NFCCG most prominently) but much more important is that he keeps putting his team in championship games in the first place.