I think Belichick let his ego get the best of him with Brady. I think he really thought that Brady wasn’t as important to the dynasty as he was. I think he thought he made Brady, not the other way around.
Brady would’ve been happy to retire a patriot if Belichick would’ve just shown him the respect he had earned. Then we’d always be wondering “who was more responsible for the dynasty, Brady or Belichick?”.
But now it’s looking like Brady was. Maybe that will change in the coming years. I’m not ready to write BB off yet. But as of now Brady is looking like the more important person in that partnership.