His main problem imo was that he became so insular. He refused to bring in new blood to the coaching staff and I believe he really thought he could just do it all, which led to the Patricia decision and other bad calls. He lost his fastball a few years before that imo, but his decision to move Patricia to OC was a career killer.
Just because BB brought back Patricia and Judge doesn't mean he wasnt bringing in new blood. Belichick was always hiring young coaches to work from the ground up. That's how he felt coaches should move up in the organization.
The list of coaches that Belichick has helped to develop into excellent coaches/coordinators is pretty long even if most haven't had success as a HC.
Below is the list of outside coaches Belichick hired during his last 3 years:
In 2021, Belichick hired guys like Billy Yates (Asst. O-line), Bo Hardegree (Asst QB) and Evan Rothstein (Offensive Analytics)
In 2022, Belichick hired Ross Douglas (WR), V'Angelo Bentley (Def. Asst) , and Tyler Hughes (Off. Asst)
In 2023, Belichick hired Adrian Klemm (OL), Will Lawing (TE), and Keith Jones (Def. Asst) .
Of the 9, only Yates and Klemm has any "direct relation" to the team. And their relation was only that they'd played here. Both got their coaching chops wet at other stops before being hired by the Pats.
Just because he didn't hire an OC or DC from outside the organization doesn't mean he refused to bring in "new blood". He didn't believe that putting people from outside the organization into positions of authority (OC/DC) was the proper way to run an organization.
I agree that BB listening to Patricia tell him that he (Patricia) could implement the Shanahan Outside Zone scheme on the offense was a career killer. Patricia allowed his ego to get in the way of getting the job done. Patricia refused to use the assets available to him. Mainly Kendrick Bourne and Trent Brown. Both of whom had experience in that scheme. It's what led to the arguments between Patricia and Bourne and Bourne landing in the doghouse.
I think that things started falling apart when Brady decided to leave after the discussions with Belichick and Kraft. I believe that Kraft, despite all his claims that he thought of Brady like a son, I believe that Kraft told Brady that an ownership stake in the Patriots after his playing career was done was a non-starter. I believe that Jonathan went and hired Wolf and Groh when it decided that they would be the drivers in the draft process. I think it didn't help BB that nearly every time he relied on Josh McDaniels for player input during the top rounds of the draft, that the Player had issues. N'Keal Harry was only the latest of the offensive skill players that McDaniels recommended that was a failure.