Russillo has been talking about a phenomenon surrounding first round QBs for a couple of years now, and it seems relevant to Mac's situation. Basically what Goff and Mayfield are doing this season is pretty unprecedented. When first round QBs switch teams, they do not go onto successful careers. You can go down the list - Jameis, Paxton Lynch, Bortles, Darnold, Rosen, Trubitsky, Wentz, Mariota, Manziel, Bridgewater, etc. - all switched teams/situations and were just as bad as they were in their former situation. They did not all of a sudden blossom once they changed scenery.
That info is interesting to me because maybe if a player doesn't show anything in those first 3-4 years - and that could be why Goff/Mayfield, players who popped for a season, are different - they're simply broken, or weren't that good in the first place. Because of that, I'm pretty confident that not only will Mac continue to be shitty on the Patriots, he's basically guaranteed to be shitty on any other team who picks him up. Barring a miraculous turnaround this year, being a backup for the rest of his career is like a high-end outcome for him.