A rare time when I am not on board with your thought process. Stafford is one of those "just good enough" QBs and I dont see how Belichick can change his fundamentals this late in Stafford's career to win a lot. Stafford can be game planned against and BB may not be able to scheme around those limitations.
As for Detroit, it totally makes sense to move on from Stafford esp from the timing perspective. If they do this correctly, they will hit a period when Rodgers is retiring, Cousins is still Cousins and Bears with a young QB. Detroit can draft a QB high, sign a vet to mentor him and hit this potential window in 2 years.