Ever since the Suh signing, which was just colossally dumb because we were more than 1 player away from Super Bowl contention (Try 10 players away), we've been on a hopeless track. We're always going to have significantly more holes than resources to fill them.
We fielded an offensive line where our depth were the guys who were hopeless failures last season, and guess what, we needed to play them and they were hopeless failures this season.
We let a starting cornerback with quickly diminished skills walk, yet traded to bring in a CB with completely diminished skills, skills that were completely overrated in the first place. We only drafted one other player for a secondary that needed a good 4 players brought in, at least. We did add another safety, but he's a middling guy at best.
Seriously, though, what's really killed this season? Offensive line, first and foremost. 30% of our defense's failures stem from the offense being unable to sustain drives and the defense wearing down quickly. Look at our points against and subtract 30% from that and then tell me how bad our defense is by the numbers alone.