I took a look at some of Brady's stats from 2010 to 2016 (e.g., DVOA, completion and INT %ages), and put them on a graph. They're normalized relative to 2010, and the vertical line represents the 2014 preseason where they drafted Garoppolo.
The key here is that there was a clear downward trend from 2010 to 2013, and then a clear rebound since then.
So, is Brady's performance the last three years the result of him "finding the fountain of youth" (i.e., something that will last for years) or a sort of "Indian summer" that will end soon?
What BB thinks is the answer to that question will go a long way in determining what the Pats do with Garoppolo.