This isn't because Belichick is any less than a Hall of Fame coach -- he is.
He's just a bad GM, and he's stubborn.
He seems to be married to the idea that, "If you want me to cook a gourmet meal, you need to let me shop for the ingredients." He's shown that he's bad at shopping for the ingredients.
Tom Brady was a sixth round pick. He got lucky falling on the pre-Mahomes GOAT.
At his age, he's not going to rebuild the Patriots.
If he takes over a team like the Chargers or, God forbid, the Cowboys, he could get another ring before he retires.
He just needs to delegate the draft and free agency process.