My view is that
The defense has a lot of talent
The defense has played poorly
Established Vets have underperformed: Brockers, Suh, Joyner, Peters, Barron (All highly Paid)
So yes Phillips and the defensive coaching staff needs to be accountable. There are some obvious issues where they had Peters playing a lot of man when he excels more as a zone guy. I don't understand why Brockers and Suh along with Donald can seem to stop the run. Joyner has been nearly invisible except for penalties and getting beat in coverage. Barron is healthy now and sure has regressed.
We should be a top 5 defense with the talent we have and for the most part we just underperform terribly. It seems to be part scheme and part not putting player where their strengths can be shown and part getting some of the high paid players to step up and do their jobs.