I heard an interesting discussion by Bill Polian on Sirius radio the other night, but he was talking about some other team (can't remember and it doesn't matter). He pointed out that the team had built its D-line around big power rushers, which puts them at a disadvantage against teams that go to a spread offense where you need speed rushers who can get to the QB quicker.
That triggered a light bulb moment for me regarding the Pats defensive line. It seems that Belichick has seen the offensive trends in the NFL and decided that his "power oriented" front seven packages were no longer ideal. I think he's made the conscious decision to replace older bigger slower players with younger faster smaller players. I suspect that Tommy Kelly was very old and very slow after his ACL repair.