Laying the blame of some sub-par defensive performances at the feet of an assistant coach is pretty narrowminded, IMO. I highly doubt Brad is rubberstamping anything, and execution has as much to do with the player's physical attributes, attitudes, aptitudes, and health as anything else.
Maybe you don't like the scheme, maybe you don't like the adjustments, but that stuff is on Brad. The fire Brad bandwagon is pretty lonely these days, regardless. If you want to get on board, be my guest.
Fans with no working knowledge of the development of gameplans, or evaluations of the execution of those plans assuming that firing a member of the coaching staff is going to fix it all is just... you can fill in the blank.
If he's staying its because Brad likes him, and isn't putting the blame at his feet. Brad has been right more often than not in his career so far. He certainly has a better idea of what the coaching staff does well and doesn't do well than anyone posting here.