I think the fall off of the defense has happened for multiple reasons. Philosophy, scheme, personnel.
Stop the run and create 3rd and long for an all out ambush don't work anymore.
You have to field enough DL (3 at least) who can keep OL off the LB's while maintaining their gap integrity, pushing the pocket when pass is recognized.
If you're going to rush the passer, you need to take away the 2 second passes. Extremely soft coverage will negate pass rush. Tighten up and force attempts at deeper routes.
We've had people in place, in the past, who could make up for other negatives because they were "that good". Those people are gone. I think there is talent there now, it just has to be used properly. Keep Shazier clean. Use more DE OLB twists to create mismatches. Keep the NT on the field and use Shazier more like a hybrid safety on passing downs. Don't get stuck by a no huddle offense with 2 DL on the field.