It's not complicated for me
McDermott took over a perpetual .500ish franchise and got them to buy into the process of becoming a good team. He was gifted a top QB and as many resources on the defensive side of the ball ask an HC could possibly ask for. During his tenure the Bills have had great regular season success punctuated by baffling playoff exits and are now fumbling toward the middle/bottom of the conference.
Nothing screams we must keep this guy at this point imo. He gave it his best shot, on to the next