You can't compare defensive numbers this year vs. any other year. This year is an abnormality. There are two QBs likely to break Marino's yardage records (one already has). I heard on the radio yesterday that in the history of the NFL prior to this year, only 23 QBs have passed for more than 4,500 yards. Right now six QBs are realistically in place to break that mark with one more possibly able to do it with a really good game. Five have already surpassed that mark (Brady, Brees, Rodgers, Manning, and Stafford), Philip Rivers is 186 yards away from 4,500 yards and Matt Ryan is 429 yards away from 4,500.
This year is going to smash the offense records for the year. Single season offensive records have already been smashed and more could be beaten this weekend (Gronk is 71 yards away from the single season receiving yardage record for TEs). If offensive numbers are going through the roof this year, that means that defensive numbers are going to equally bad. This will probably be the statistically worse year for defense for the league.
This is why I am not nearly as concerned about the Pats' defense as many others are.