It isn't that McDermott is so terrible, by himself. It's that he is constantly married to an offensive staff that is wholly and entirely incapable of evaluating or coaching. They are seeking out and retaining players on offense that are terrible and they have traded away or let players go that were far better than anything they've replaced them with. The promise was that everyone they were replacing was replaceable and inferior and that bringing in different players was the answer. The truth is that the coaching on offense is sub par and that each player they let go or replaced only increased the talent deficit until we now have the most inept offense in the NFL and it's not even close.