As fan base we've been given a glimpse of Brady's exhaustive commitment to maintaining throwing mechanics, core/body strength- especially the legs, flexibility and conditioning. I have no doubt that to play the # of years - and at the level he has, PM has kept himself in tip-top shape as well. However, things just start to go. PM doesn't chuck the ball all over the field as he did when he was 25 or even 35 years old. If you watch all the great QBs in their mid/late 30s..Montana, Marino, Young, Favre, Elway they throw with just a little bit less zip than they did when they were younger.
I'm in the camp that believes that PM has lost something on his throws and it's been 3-4 years in the process of it becoming noticeable on a consistent basis. BB has said before its almost impossible to predict when a player is done as everyone is different. For some, it just happens. For others, its more gradual.
Elway needs to determine the root-cause of his performance issue. Once he does that, he can make an educated decision on how to proceed. Either way, I have to assume he is leaning towards restructuring his deal so he can sign a quality backup in case PM continues he steep decline.