People forget how badly Brady played for Brady in 2019. He looked like the end was nearing back then. We have the benefit of hindsight from his production last year to know now that was just poor talent around him and Brady not putting in the effort in anymore because he was unhappy with his situation. But since 2007 when started to put up production as a modern era QB, 2019 was either his worst or second worst year in terms yards, TDs, completion percentage, and yards per attempt.
There is a reason why teams were not falling over each other to sign Brady when he got free and the Bucs had little or no competition for him. Belichick wasn't the only one who thought Brady may be hitting the age wall.
I don't think Belichick just said that the guy was 41 and he is bound to break down at any time now. There were actually clear signs that he was breaking down. Unfortunately, they didn't have to do with his age and more about his motivation and talent around him.