I’d take Fisher as a defensive coordinator all day. His best defense was when he was running it. I think he took over for Blake Williams mid season.
I've always felt like Fish was promoted to HC too soon, that he missed out on some key formative years where he would have honed his schematic knowledge. For me at least if I'm a GM there's no way I interview a guy for the head job if he hasn't excelled above his peers at the coordinator level. Because no matter how well you interview, or how good a dude you are, you're still lacking that experience which is crucial in coaching your coaches.
Belichick had a long history of top defenses. Fish didn't. McVay had that history of productive offenses at the coordinator level, a short history but his impact was felt very much in the time he coordinated. IMO that's something the GMs are missing in these interviews. If you bring in ten guys who have a shortage of demonstrated excellence and experience at the coordinator level, there may be a guy in there who can overcome that. But I think it's a bad way to go personally.
Basically I think that guys like Fish who are really smart and good with people can get themselves hired quicker than they should. By killing the interview. But then when they get their jobs they require top coordinators on BOTH sides and that's something you cannot afford to have in today's NFL at the head coach spot. He has to be able to hold down one side of the ball due to poaching when you do start winning.