As I've said before...
At this point, he had to go. But what Belichick did to this kid's potential was unconscionable. After a pretty successful rookie year, it looks like NOTHING was done to help his second year QB, arguably the most important year in determining a young QB's trajectory. There was always going to be a point where defensive coaches had his early tendencies figured out. It is then up to the coaches to work beyond them.
And I still say that what Belichick did in his second year was worth Belichick's job. The sheer arrogance of putting Patricia and Judge in charge of his second year quarterback's development was unprecedented. Somewhere along the line, something eroded Belichick's ability to be self-critical.
It really sucks to say, but even guys who have incredible football knowledge, who can diagram, diagnose plays with ease... that at some point, they lose the ability to 'coach'. Coaching is not just about smarts. If it were, Belichick would still be here.
Further, I think to be a Dynasty coach, you have to have a lot go right. The right players, the right ownership, the right time. If you get that, you start building belief. That can help you endure when you start losing those 'right' pieces. And it's hard work to keep all that's right...right. You get older, you stop wanting to put all that work in. And it seems Belichick didn't want to anymore. To be sure, he was still putting in the hours. But he didn't challenge himself as much anymore. By the sixth Super Bowl, it's hard to blame him for thinking he had earned the right to not be challenged. But that's not football. There's been more than a few successful coaches who have just retired because they couldn't do the thing anymore. John Madden, probably most notably. Some guys stick around because it's all they know... IE: Shula, and now Belichick.