No doubt. There is only a finite amount of time you can expect anyone to cover anyone in the NFL - particularly once the play breaks down (and with the ever increasing trend of mobile quarterbacks)
But, the Pats have almost always been an adequate D rather than off the hook - Ravens/Broncos. And this season, while they haven't generated a crazy amount of pressure on the QB
The following teams have equal or less sacks than them -
Texans
Rams
Bengals
Lions
Niners
Jets
Chargers
Saints
Raiders
Colts
Cowboys
Browns
So they are mediocre rather than completely shit up front
You then take into account Bill deciding - Chandler Jones (brought a ton of pressure last season and had 10+ sacks) was dispensable in the off season and then Jamie Collins (for a whole host of reasons) with the final nail in the coffin being, Malcolm Butler not playing as well at the back.
I just think, the Bill always looks at his D to simply not lose them a game (bend but don't break).