Not picking on you in particular, but why do people think this? Fitz is a fun guy to root for, but what about him makes him a good coach?
He was not a great teammate after Tua took his job. He doesn't have great fundamentals. He is hardly a technician of QBing. What makes Fitz "good" is his playing style and personality. Guys gravitate to him because he plays so hard and its a great dude.
Could he be a good coach? Maybe, but just because he's been around for a long time doesn't mean he will be a good coach.