First off, the answer for any player is "maybe." It's a hard game.
Brady's freakish will power might seem to argue that he'll make the play-to-50 thing work, come Hell or high water, but a guy with that kind of will could also decide one day, in the face of all expectations, most of which he has built - thinking of his wife's wishes, of his kids, of his health, of evolving interests in other areas of life, whatever - to do what a weak-willed person would be too weak to do: change his mind.
I have a funny feeling that's just what he'll do, and I wouldn't be surprised if he did so this coming year. It makes sense in a lot of ways, and in addition to the willpower, there is in Brady an overlooked coldly logical (Belichickian!) element which just might prevail.
I hope it doesn't happen, but it's a pretty cool narrative in its way, so I can dig it.