I realize Bills fans are salty for being the Patriots' plaything for 2 decades and want to discredit everything the Patriots did at any opportunity, but Belichick is an excellent coach* (and was initially also a good GM). It wasn't all Brady.
Good coaches adapt to their players available (never had an elite WR or RB except a couple seasons with Moss, varying between running teams, spread 5 offenses, and heavy 12 personnel. I mean, look at the run-only game against the Bills --- and it worked!). On the defensive side, lights out. He destroyed the Greatest Show on Turf (remember, Vinatieri had 3 GW Super Bowl kicks, but two times that's the defense, not Brady), owning Peyton Manning, etc.
As a GM, early on he was very good, later.... not so much. But the key thing for Belichick was the ability to move players the year before they slipped, or at their highest value. Richard Seymour comes to mind. Keep the winning going or get tied to your veterans? As a GM, he was cutthroat for awhile there, and it worked.
*Show me an all-time great coach who didn't have at least a competent QB -- and don't offer up one-hit wonders like whoever was coaching Trent Dilfer (who was an average QB) when the Ravens won that one Super Bowl.