Your theory that Tom would be struggling is completely unfounded. He's 43 for Christ's sake and he has had to get acclimated to an entirely new team, coaches and players. He's a borderline pro bowler. What he's doing is unprecedented and incredible.
Belichick looks like a total fool for not doing everything possible to have Brady finish out his career in NE. The Patriots are a total disaster right now and their future looks very bleak, not only because they have a rather untalented roster, and no plan whatsoever at quarterback, but two teams in their own division are skyrocketing past them. Not to mention several other teams in the conference will be leaving them in the dust for the next several seasons.
Look, for at least this season, had Brady remained their starter, they would be cruising toward another division title, even with the otherwise putrid roster.