This. This is the answer. Brady was not the QB who could lead the new, revised Patriots offense. They are changing things up and need mobility. No money to pay him, and really no desire to have him back in my opinion.
Cam is a low risk, high reward gamble. If he is good Cam, great. If not, you play Stidham, which was already your plan. Cam fits the style of play going forward. Mobile QB with two TE, or one plus a FB or H-back, running run pass option schemes, multiple play action, motion sets. Dan Orlovsky tweeted as much today, comparing the Patriots look on offense to the Ravens.
I think Bill has wanted to make this change for some time but it wouldn't work with Brady. Would waste Brady's skill set.