Martz effected the style of play in the NFL. And I am not a fan of how he handled the Rams when DV left.
Kelly just gimmicked his way to one decent year, then gimmicked his way to the unemployment line. He's kind of an example of good old boys network if you know what I mean.
However your main point is IMO fair. There are a lot of recycled coaches and I hate the good old boys network. Each year there are several HC/OC/DC openings. Those openings happen for very good reason, the failure rate is very high. I'll use Linhan and Spagnuolo as an example. Both poor HC's but Linehan got another shot at OC and that's where he belongs and has been successful. Spagnuolo also got another shot as DC and stunk on ice, then he got brought into BAL where he had some history since he was there for a couple of years. They dumped him in a year and the Giants hired him back and finished last on defense, just like the Saints did. One example is successful and the other is not. Yet it happened!!! Drives me crazy. There are WAY too many examples for my liking.
I'm not a big proponent of bring a college HC in to be an NFL HC but I would find OC's and DC's in the college ranks and give them a shot at OC/DC and maybe see if they end up developing into a potential candidate before I would give a guy who flamed out two or even three times. I know a lot of the big time programs have HC's that make lots of money and have more stability than some HC's but that doesn't mean the assistants don't have talent.