Well it was Shanahan's zone read system that made Davis the player he was too. They went in the opposite direction of the rest of the NFL and got fast, agile lineman. Mike Anderson, Olandis Gary and Clinton Portis (who Shanny somehow flipped for elite CB Champ Bailey in 1 of the worst deals the Redskins ever made) all had a lot of success in Denver's system too. But they didn't have Elway anymore and they didn't win more than the occasional playoff game every other year post-Elway (to be fair, they didn't lose to chopped liver either, but Shanny from '00-08 only won like 3 playoff games)
That's the biggest reason it looks like Belichick is on his way to standing the test of time too though. Because there is no Belichick system, there is no west coast offense, zone blocking scheme, zone blitz, Air Coryell, Air Raid, Greatest Show on Turf, Tampa 2, Seattle cover 3 etc etc...All this stuff has to evolve over time
His system is adaptability, playing lefthanded, righthanded, to your strengths, to attack opponents weaknesses, run heavy, pass heavy. There is nothing to "figure out"
That's also why his assistants mostly suck in new jobs too, cause they don't really have anything to bring elsewhere and being an authoritarian dick when you've won 0 rings as HC doesn't really work. Flores seems to understand this pretty well and is trying to forge his own way, whereas McDaniel, Patricia thought they were the son of a untouchable mob boss.