This is just a personal theory, but I wonder if the biggest factor in Belichick’s downfall is the current core of players refusing to be coached ‘tough’.
I’ve seen multiple accounts where former players stated that Belichick would chew out Brady as much as the fringe rookies. Because Brady accepted that only perfection was acceptable, there was no excuse for the rest of the roster.
Without Brady leading by example, the team has switched off.