Brady himself said "I have more to prove". I always thought he meant by this that he would like to show the world that he can win SB without BB.
If in fact he did win a SB without BB, it would forever change the narrative that he is a "system QB". I do believe a part of Bardy wants to finish by proving this one final point about his career.
Could also backfire, because if he leaves the Pats and stinks up the joint, that will add more fuel to the "Its all BB" fire.
Beating Ptrick Mahomes and the Chiefs as a member of the Raiders to win the AFC West and the AFC championship seems like a stretch. I do not think he is afraid of taking this type of risk.