It's hard to break a defense down by units and say "it's all their fault." Each level affects the other. If a QB has a monster draped all over him 2.7 sec after the ball is snapped, people may assume the secondary is awesome when, perhaps, they couldn't cover grandma. We know that line was very talented last year, but Bosa praised Coach Johnson a while back for implementing a rotation because "we were exhausted at times last year." The LB corps was thin last year. No getting around that, but there were problems with them reading pass and getting back to their proper depth. I suspected that a lot of coverage busts were due to the DB's not trusting the LB's to be where they were supposed to be, and trying to do their job plus cover for the LB's. It was really a cluster of dysfunction.
But part of it was definitely coaching/scheme. Any good defense has the characteristic of being aggressive. Against the pass, those guys were too often the 3 man rush, 12 foot cushion type of defense. I get it that when your offense is hanging 50 on people, you can afford to play softer, but at some point you get into a game where you really have to stop someone. I can take some bad defense if I have to, but I want aggressive bad, not passive bad. I don't want to be throwing up in my mouth a little like I had to at times last year.