Brady is great, but we've never seen Belichick coach a team with a good or very good QB not named Brady. Most of the QBs that have started under Bill are not that impressive. Drew Bledsoe is the best of the bunch, but by the time Belichick became the HC, Bledsoe wasn't the same Bledsoe. Bill winning a playoff game with perenial loser Vinny Testaverde is an accomplishment. Making the playoffs with rookie Mac Jones is an accoplishment. Winning 11 games with Matt Cassel is an accomplishment.
Don Meredith, Craig Morton and Danny White are far better QBs than the ones Belichick coached not named Tom Brady.