I expect 'definitely' is a difficult assessment of anyone's future in the NFL, and more so with Brady.
Why is JAG here? He may not have received the basket of first rounders expected for an NFL trained second rounder (despite all of the claims), thus never received the value he represents as insurance (and maybe Brissett is not ready to step in now in the coach's eyes). Or maybe he is Brady's exit strategy. It would have been more telling if the Pats had to pay him this year, or extended him. Right now, JAG leaves next year.
As for Brady, I still believe he leaves the game before he needs to be moved. After the Super Bowl, it is tough to believe JAG (or anybody) would represent a value replacement (price tag and return on investment) for Brady. Is that this year? Who knows?
I suspect not even Belichick, who seems to have a sense of performance dips before the big deal, has that answer because Brady, to date, has not aged like typical QBs. In 2014, after KC, he was old and on his way out. Then not. In 2015, Denver beat the crap out of him thanks to the line play, but somehow he kept the Pats in that game after a brutal beating (JAG lasted 1.5 games, freak injury or not, so do you see Belichick betting the farm on him weathering the storm like Brady?). Last year, it was he only played 12 games, not 16. So do you really want to bet on him not being great, and the greatest for the Pats, in 2 or 3 years? I wouldn't, and that would seem to be a reach from Belichick absent tangible signs Brady has the skills to work the offense as effectively as a replacement.