A lot can happen over the course of a season, so it's kinda pointless to entertain the what-ifs after the very first week, especially considering that the scenario below is being very optimistic based on just one game that he's started. Nevertheless...
If Garopollo goes 4-0 (or 3-1 like Sneak predicts), showing leadership and delivering 3 or 4 big wins without Brady and the rest of the guys who're either injured or sidelined for whatever reason, and Brady comes back and the team gets healthy, starts clicking again, going to another SB... what happens with Garopollo? His contract is up at the end of the season, but Brady could very well play 2-3 more seasons. You gotta figure that although he would love to be the guy in NE, he's obviously playing second-fiddle as long as Brady maintains his health and his level of play. There would definitely be a market for him as a starter somewhere, and depending on how well he performs given this opportunity to show what he has to offer - he could very well stand to gain from a handsome contract offer from another team.
It'll be interesting to see how things develop this year in NE, especially if they get back to the SB and Brady has another Brady-esuqe type season. I'm sure Belichick would love to find himself in that situation, and I wouldn't bet against it. But it could produce quite the quandary for NE if everything goes as planned, because I don't see a promising QB settling for a backup role for less money in this current QB-light market.