This is my opinion, and I would love to have more information to back it up, but here goes:
I think the problem with our defense this year is three fold, a sort of perfect storm. First, we have a lot of older players, second, we are short on depth, and three a lot of really good backups are showing that they aren't good starters. Bear with me as I flesh this out.
Depth/age problems go together, because they compound each other. Those older players need fewer plays, and this year with injuries and shallow depth they aren't getting spelled as much. As for good reserves not being good starters, I think it's obvious where I'm going with that.
The Defensive line is obvious, we've been pretty well crushed with Injuries there and only the fast development of youngster Ziggy Hood and Keisel's awesome season has saved us. Last year Aaron Smith, Keisel, Kirschke and Eason were healthy most of the year and rotated a lot.
Safety: with Troy out it is Carter Clark and Mundy. Carter is solid in spot relief or as a dime defensive back where we can choose his match-up and limit his responsibilities. No one has four starter quality safeties on their roster and Carter fits as a good back-up. He is not a good enough starter for our expectations on defense, and since he is replacing Troy it is even worse. Carter is a starter level player, but not a great one. Mundy is a good find, and solid depth, hopefully he can continue to improve and become more than that.
Cornerback: Ike is having an off year, he seems to be an up and down guy, last year was a solid one, this year not as good. Gay was a great third corner last year. Having three corners playing at a high level is a big deal in today's NFL, and Gay gave us that last year. He also was a good compliment to McFadden who was a very physical corner while Gay was better at coverage. With McFadden we could choose our coverage allowing Gay to face smaller players or cover slot routes where quick slants aren't as hard to cover as they are outside. Without McFadden Gay doesn't get that luxury, and thus one of the best 3rd corners in the league turns into a lousy 2nd corner. McFadden has also struggled with an increased role for Arizona, so perhaps they complimented each other even more than I give them credit for. Last year Deshea was our fourth corner, and got on the field in the dime, alternating with Carter depending on what we were doing. This year he's played third CB a decent bit with Carter starting and no depth at corner. In case you didn't know Deshea is old. Give the FO credit, they saw the problem and drafted 2 corners while bringing in Ratliff and R.Lewis to try to replace McFadden in the short term. None of it has worked out to date. Let's hope the young Corners can swim when they get thrown in the water against the Browns.
Linebacker is a bit tougher. I don't know how much, but I think Woods or Bailey got some PT at OLB last year, but I'm not sure, don't know if there is a place to find that out. Mostly though Timmons spelled Woodley and Harrison last year while also spending time in the middle spelling Foote or Farrior. With Foote gone Timmons does not play outside at all and Farrior doesn't get much time off, as both Fox and Timmons are more mistake prone than Foote or Farrior. Farrior looks older this year, and he is getting abused at the end of games, if that is a from losing a step, being worn down or different defensive schemes I don't know, but I wouldn't be surprised if it was a combo of the first two. I don't think it is a secret that holding isn't being called on outside LB's this year, and that certainly doesn't help the situation at all.
I don't believe causes are excuses, so I don't think I'm letting anyone off the hook for the decline, I just think the decline is a realistic outcome of injuries and lost depth. We are an old defense that is starting to integrate new players. Our awesome quality of depth the last several years was a huge part of our success, but it kept young players off the roster. I'll take that for two Super Bowls. But now the transition has begun, and the quality has definitely dropped. Hopefully Sonny and Ziggy, Burnett and K.Lewis develop into quality players, and hopefully we find a NT, some OLB depth (a youngster to develop behind Harrison would be awesome) and please please please a safety that is fast, can cover and tackle. I hear Berry is going to be a huge bust if any opposing FO's are reading this.
I've been calm about this season mostly because it is my opinion that the core of the teams that won those SuperBowls (Ward, Parker, and a bunch of the defense) are declining, and that team's Super Bowl window is done. I'd love to be wrong, but I'm happy with two Championships from those guys. We have some pieces in place to build our next great defense with, and the offense looks to be in pretty good shape for a while to come, we won't be off the top spot in the AFC North very long at all, and then the Steelers will be back on top where they belong.