That isn't true. In the playoffs in the 2001 season, the defense gave up 13, 17, and 17 points. You are going to tell me that no other QB could score at least 14, 18, and 18 points? In those games. In fact, Bledsoe scored one of the TDs vs Pittsburgh. Also, the only TD scored on offense. The Pats scored more on special teams on that game (2 TDs).
And in the last nine games of the regular season the Pats gave up 10 pts, 11 pts, 24 pts (loss to the Rams), 17 pts, 17 pts, 16 pts, 16 pts, 9 pts, 13 pts, and 6 pts. You telling me there was no QB in the league was capable of scoring 17-20 PPG? Sure with another QB the defensive points allowed could have changed, but if you have a top QB, not significantly.
Anyone who doesn't accept that the defense was the biggest reason the Pats won in 2001 aren't dealing with reality. The Pats had the best defense in the league the second half of that season and in the Super Bowl.