Belichick without Brady is a career 8 games below .500 with all of 1 playoff victory in 7 years. With Brady, he's a "genius". Without Brady, not so much.
That's when he was at Cleveland, one of the black hole teams in the NFL. Nobody wins much at Cleveland. But even there he had an 11-5 season, with Vinny Testaverde and Mark Rypien, neither of which was ever an elite quarterback, Rypien did win a SB with the Commanders but with running backs that cumulatively got over 2,000 yards, and receivers like Art Monk, Gary Clark, etc. he had tons of weapons, as well as the "Hogs", and Joe Gibbs as the coach. Rypien never did much before or after that SB season.
Really I think it's a "perfect storm" situation, you can't run a precision offense without a precision quarterback, have to be able to execute the plays. And even a super accurate quarterback needs open receivers, who have good plays to run. Add to that Brady's durability, the weak division schedules so many years and of course a good dose of luck you get what you get.
Just think what the 70's and early 80's Cowboys could have done if Roger hadn't had to retire early, and of course not having to wait 4 years to get to the NFL, could have equaled Brady and Belichick's record. But that didn't happen...