Compare the Patriots with any other organiztion Jimmy G could wind up starting for.
I can see good reasons to go get a gig now if possible, but there's plenty of reasons for Jimmy G to be patient too. It really depends on whether BB is willing to give up on TB12 a year early like he has for so many other stars. If he is, then give Brady a hard limit of 2018 as the end of his Patriots career and turn the reins over to Jimmy G. A few wilderness years for Brady won't tarnish his reputation too much (and I think Brady would be too smart to sign with the organizations that would really be a FU to us if he signed with, like the Jest) and transitioning to the younger model to keep the dyasty alive is a move he'd probably understand even if he didn't like it.
This is an incredibly skewed, biased post, and this is not how players think at all.
Pro athletes do not look at franchises and think, "man, that place is a cesspool, if I go there it will ruin my career." Pro athletes are arrogant and prideful, and they think, "yeah, that organization sucks, but I'm so good that I could go there, be there savior, and turn it around." If what you were saying is true, the Browns/Rams/Jets/etc would never sign free agents. No matter how terrible a franchise is, money talks.
If Jimmy G is seriously going to re-up (there's no chance in hell that happens, by the way, I'm just humoring this for the sake of discussion) with Brady stating that intends to mid until his mid-40's, then I would seriously question Jimmy's drive and desire to win.
Barring a catastrophic injury or decline in play with Brady, Jimmy is as good as gone. It doesn't make sense from Jimmy's standpoint and it doesn't make sense from the Pats standpoint (committing that much money to a backup QB, when they could spend it improving the roster elsewhere).