I also am not 100% convinced they can win a championship but I just get the feeling that people think the Packers, Lions, Giants, Jets etc. have drastically better defenses than the Pats...just isnt true.
First of all Green Bay's defense isn't good this year, they've been pretty bad actually only a lot of turnovers have kept them around average. Turnovers are something that's tough to rely on that much as a defense.
NE is a worse defense than the others you name, you can't give up 6.1 yards per play (29th in the league) and be as good of defense as Lions/Giants/Jets/Bears.
They give up 7.5 YPA on passes (26th)
They give up 4.3 YPA on rushes (18th)
They give up 4 red scoring opportunities per game (29th)
They give up TDs 50% of the time in the red zone (16th)
They allow for third down conversion 43% of the time (28th)
They definitely have bended and not broken too much, but you can't count on giving up so many yards and scoring chances as it'll eventually bite you.