2. Khalil Mack is still an elite run defender
While Khalil Mack's pass-rushing numbers have gone down in the last two years what hasn't changed is the fact that he is one of the very best run defenders in the entire league on the outside. While the main focus for the Chargers is on the interior defensive line, the Bolts need to build a complete group and Mack is the best option the team could have gotten to defend the run on the edge.
Even if Mack does not return to the same pass-rushing player he was three years ago (90% of what he was three years ago still gives the Chargers an elite pairing), he will be able to contribute against the run at a very high level that has a ripple effect on the rest of the defense.
There is a route the Chargers can go with the defensive line that would create a wall for stopping the run while having elite pass-rushers on the outside. If the Chargers do create this wall, Brandon Staley could better execute his vision of playing fewer players in the box and stacking the secondary.
That is the key to unlocking this defense. The Bolts would still need to build out the secondary but if the Chargers can just be above-average in stopping the run without stacking the box then that allows them to shut down the passing game with more defensive backs. The team did not have the personnel for this vision and Mack is a key first step in better executing the vision.