Where Bill's genius came into play most was as a GM convincing Brady to get paid well below levels of the other best QBs in the league allowing Bill to build SB winning rosters under the salary cap rules
When you speak to NBA historians, those in the league or who have covered the league for decades, in commenting on Belichick the GM and coach, they say Belichick is the best ever because he could do what he did under rules(draft, salary cap, 32 team league) that were there to promote parity in the league and prevent teams from being the best in the league for long periods of time.
And just about every other team in the league has gone up and down the power rankings over those 20 years except the Patriots. You couldn't do that without the best GM, the best coach and the best QB, who happened to be massively underpaid, during that time period.