There is a couple things in play here in my mind:
1. Not everything was right in the state of Massholio. Brady is willing to finish his career in NFL Siberia over signing friendly and finishing with Pats. Suddenly, Gronk wants to play football again. He alluded to not having fun anymore in NE at the end of his career.
2. They will probably have a good team and if there is a season they will certainly be pre-season darlings. But they won't be getting the Pats breaks. They will get more than they did, but not that many. And Arians will have to say, you run the offense Tom. Cause his offense and route combinations will not mesh with Brady's pop gun arm.