People are sort of ignoring less black/white possibilities. I assume that is deliberate.
Lets say Fitz started as a decent defensive coach, learning as he goes. He hires Hankwitz, who is a wizard. The defense plays well for a long time. Then the wizard leaves and Fitzgerald hires a fool. Biggest mistake of his coaching career. The defense is suddenly a joke. People declare Fitzgerald had nothing to do with the prior years of defensive prowess. Fitzgerald fires the fool and brings in a promising young wizard apprentice. Great hire. The defense bounces back to respectability, despite losing a few players due to administrative malfeasance, not to mention losing Fitzgerald himself.
It isn't too unlike Chris Collins situation 18 months ago. He hired a defensive wizard and that probably saved his job. Had he hired a fool, he'd certainly be gone. And, like Fitzgerald, his teams have tended to be better defensively, with obvious limitations on offense.
It is also interesting to me that people can trash the offense and ignore the impact of losing Ben Bryant to injury. Losing your starting QB is almost always crippling for your offense. Bryant is (or was) at least as good as Peyton Ramsey. Sullivan is not. My guess is that there is a very high correlation between the Fitz-trashers and the people who pretend Sullivan is almost as good as Bryant, because that enables them to continue trashing. Of course that requires ignoring the possibility that Skip Holtz is even worse than Fitzgerald.
At some point the trashing has to be supported by some logical set of facts. And most of it isn't.