Rex is a legitimate defensive coordinator, and one of the best in the league. Herm was a clown and Mangini was a headmistress.
The Jets passing defense was near the bottom of the league (#29) when Rex took over the team, and in his first season the Jets were ranked #1 in that category (as well as #1 in total defense). They've remained a top unit in that regard during his entire tenure including this season, in which he's lost Revis, where the Jets currently rank #6. The man knows defense. People talk about this being a passing league in regards to our less-than-stellar offense, but its a passing league when the defense is on the field, too, and in that category Rex clearly excels.
Also, I don't buy the idea that Rex doesn't want to throw the football. What options does he have in that regard? He talks up the ground & pound largely because that's his best option. And he's toned that down as that option has declined. But he let Schottenheimer try to open it up with Sanchez, and we've seen Sparano throw the ball a lot in the opener - until Sanchez fell back to earth hard.
The real decline on this team is personnel. That's supposed to be Tannenbaum's job. People talk about his ability to find cap room, but he created his own need to find it by having Sanchez, Holmes, Harris, Scott, and Pace making up a full 25% of his cap allotment. Trading picks, overpaying free agents, and drafting poorly in general. That's the major problem on this team.
If the Jets were to unload Rex, he'll be able to wave off coordinator offers while he waits for another head coaching job - if he so chooses.
Yards/game allowed by defense during Rex's tenure:
2009-252.3
2010-291.5
2011-312.1
2012-359.8
Maybe it is me, but I see a negative trend there
