Totally untrue. If it was true Mangini would still have a job. And your comment that "He hasn't had it for years". Did you live on another planet last year when the Packers went 13-3?
I think its the rash decision making/lack of direction that has caused the mediocrity. Maybe I am way off base here, but when I apply my line of thinking (regarding the Jets & Mangini) to my own life and my own career I see the value in patience. I have been blessed with some jobs in my career where my employer was patient and gave me time to develop. They believed in me when they hired me and were with me even when other employers may have fired me for not coming up to speed quick enough. In the end I believe I was/am an asset to my company(s). Had they dumped me for a young college guy with a fancier degree we all would have been worse off. Well maybe not the new guy, but thats not the point. I have seen this played out time and time again with co workers as well.
Fisher had taken a team to the SB that went into decline and was in cap hell. They bit the bullit and rebuilt the team. Fisher had already proven he was a top NFL HC. Mangini hasn't proven squat and pretty much lost this team after a dissaster last year.