You know bellichek is a defensive guy, right?
The pats change offensive assistants more often than the kardashians change husbands.
Belichik hasn't been solely a "defensive guy" for more than a decade. He has his hands on everything with that team. He's even gone whole seasons without an offensive coordinator. It's clearly his team, as assistants come and go., never achieving anything close to the same level of success elsewhere as they have working under Belichik, with him pulling the strings.