Pats are just up against it now next year with Jimmy and Brady. No matter what route they go, they'll be faced with a tough choice. Brady will be 41, and iirc, no QB in the history of the sport has remained elite at that age. Of course, Brady is the GOAT, so, who knows. Point is, it would be quite the gamble for the Pats to put their future on the line starting a 41 year old QB with no locked-and-loaded #2 QB waiting in the wings, but to keep Jimmy G next year they'll either have to franchise-tag a clipboard holder or give a clipboard holder a starter-caliber deal.
Brissett hasn't gotten Jimmy's hype, but he seemed good enough to be a viable, winnable stop-gap starter in the event Brady's play suddenly drops off a cliff in the future. Now the Pats don't have that option. If they let Jimmy go next offseason, they'll be all-in on Brady lasting long enough in his 40s to give them enough time to develop another successor good enough to QB this team with a roster poised to continue competing for silverware every year.
This has happened before, sort of. Niner fans never thought Montana would retire wearing another team's jersey, but Steve Young was ready. More recently, Favre-Rodgers in Green Bay. Both instances, the team allowed a still effective, first-ballot HoFer to go to another team because they believed the young guy they had waiting in the wings was too valuable to let go.