Belichick has been a head coach without Brady for 7 seasons, this is his 8th. He missed the playoffs for all but one year, and has a record of 56 wins and 65 losses, on his way to missing the playoffs for the 7th time in 8 years without Brady. 5 losing seasons, very likely 6 after this year.
Belichick has been a head coach with Brady as a starter for 18 years. Made the playoffs 17 times, 9-7 record the one year they didn't, 9 SB appearances, 6 wins, a record of 218 wins and 62 losses.
The proof is absolutely in the pudding. He's a really good coach. Brady is the GOAT. Give Brady anything but trash and you have an instant contender. The Pats were in it every single year since day 1 after he became the starter.