It's absolutely an excellent argument; it sounds just as good as yours does (i.e. worthless), it's just as easy to prove as yours is (i.e. impossible), and that's the point.
Not because of anything wrong with either Brady or Rockne, but because of the complete futility of trying to say who's the best ever. (Especially if the true best ever was a cornerback or offensive tackle)
This is certainly a thing.
JJ Watt, before the injuries set in, was so ridiculously good that you basically can't even compare him to other defensive players at the time. If there was a stat, he dominated it, and he passed the eye test as well. He defended the run, he rushed the passer, he caused turnovers and he racked up tackles. Darrel Revis, during his 4 or so year peak, was as disruptive as one can be at corner. He was so good that he didn't pick off all that many passes, simply because QBs never had an opening on a man he was defending, but advanced stats and play-by-play tracking showed him to be definitively, massively better than everyone else, and again, that passed the eye test. Dwight Stephenson, when he was center for the Dolphins, was so ridiculous that, despite being small, nose tackles would break their own schemes to shade off him to not be embarrassed.
That said, the QB is the most impactful position on the field/court/rink for any large team sport. They literally touch the ball on nearly half the plays (all offense plays except tricks, and most teams don't run more than a couple of those a season at most), and are responsable for most of the active decision-making on most of those plays. That's just not true for any other sport. That doesn't mean that the QB is the best player automatically, but it is hard to argue against the value of someone who touches the ball that often. Skill, with everything that encompasses - talent, availability, leadership (which is a ******** dump, but I digress), etc., is part of the "value" equation, but the other part is opportunity. And the QB has many more opportunities to impact a game, positively or negatively, than any other single player in a team sport, compared to the opportunities of those around them. And those opps are visible, because the QB has the ball.