I used to be on the side of 'it's Belichick's defenses, not Brady' for much of the last 20 years, but since they split, I've changed my view. Belichick has just never done anything as a HC without Brady, though we all accept he can coach defense. Also, his 'coaching tree' has produced such rancid, rotten fruit, that one can't help but think that the fabled 'Patiot Way' was really the 'Brady way'.
It was obviously finding a generational QB in the draft more than anything else, like the 49ers drafting Montana in 1979 rd 3 or the Cowboys drafting Staubach in 1964 rd 10 knowing he had mandatory Naval service. There's likely no Tom Landry or Bill Walsh legend status without those guys, same applies to pretty much any coach. Joe Gibbs managed to win championships with three different starting QBs, perhaps he's the true GOAT of coaching. Andy Reid probably counts his blessings every night for the team drafting Mahomes, and he's a damn good coach. Then you have guys like Jimmy Johnson and Bill Parcells that truly built teams and could coach a pickle to perform.
I believe everyone can put to rest the Belichick is the GOAT and responsible for most of NE's success. It was clearly Brady and the 2020 Tampa Bay championship should have put it all to rest but we have further proof in the disaster that is NE post-Brady. That's not to take away from Belichick's coaching and player personnel moves, espcially his defensive genius, though the personnel part should be largely/partially credited to persons like Scott Pioli, Thomas Dimitroff, Nick Caserio, Bob Quinn, Matt Groh, et al not just Belichick. And special mention for spygate those early years Patriots and probably for the SB win against St Louis that Brady started.
Belichick is 63-77 without Brady on the roster, including 2-8 this year, with what will be 7 losing seasons in those 9 coaching years; 1-2 in the playoffs, 0-1 in NE. That's not accounting for the 5-11 record in 2000 with Bledsoe making 16 starts, nor the 0-2 mark in 2001 when Brady came off the bench for an injured Bledsoe on the final drive of the second game (Brady then going 11-3 as the starter), nor the 11-5 mark NE posted in 2008 after Brady was lost in the first game (Cassell subbing in a 0-0 game late in the first quarter), nor the 3-1 start in 2016 when Brady was ridiculously suspended. Those games would make Belichick 81-96 with someone other than Brady starting (or 82 wins for the Cassell 0-0 sub-in game if you want). That's Jerry Glanville, Joe Philbin, and Dick Nolan winning percentage neighborhood; Glanville went 3-4 in the playoffs and Nolan was 2-3. But he's 219-64 and 30-11 playoffs with Brady starting the game, gee wonder why that is.
Including the other teams mentioned above, these are the for point totals for Belichick's non-Brady starting teams, which included defensive and special teams scores further reducing offensive output. I've also added the QB starts for that season. Scoring was different in the 90s but those Cleveland teams were hovering just below/just above average ouput except for 1994.
https://www.pro-football-reference.com/years/NFL/scoring.htm
1991 Cleveland - 293 - Kosar 16
1992 Cleveland - 272 - Tomczak 8, Kosar 7, Philcox 1
1993 Cleveland - 304 - Kosar 6, Testeverde 6, Philcox 4
1994 Cleveland - 340 - Testeverde 13, Rypien 3
1995 Cleveland - 289 - Testaverde 12, Zeier 4
2000 New England - 338 - Bledsoe 16
2008 New England - 410 - Cassell 15, Brady 1
2016 New England - 81 - Brissett 2, Garoppolo 2 - first four games only
2020 New England - 326 - Newton 15, Hoyer 1
2021 New England - 462 - Jones 17
2022 New England - 364 - Jones 14, Zappe 2, Hoyer 1
2023 New England - 141 - Jones 10 - on pace for 240