Playing in 4 super bowls and winning 3 was obviously incredible. After that, Kraft and BB could have made the decision to keep Brady no matter what. They decided to not do everything to keep him, which could have included franchising him or manipulating the cap to give him the extension he wanted.
It was a super difficult decision. They thought they were doing the right thing. With 20;20 hindsight it was a bad decision that has resulted in putting the patriots into the sinkhole of mediocrity with no easy escape.
3 years out from Brady's departure, it appears to me that BB may have possibly underestimated the difficulty of rebuilding post Brady. Some of this may have been overconfidence on BB's part based on the unprecedented success he had while Brady was here.
With 6 or 7 top flight QB's in AFC, not at all sure Pats are on an "upward trajectory". If you simply look at facts and data, without Brady, BB is an average coach and bad GM....so it's difficult to have confidence they will be on an upward trajectory while Bill is at the helm in his 70's.