Brady's decision was incredibly straightforward and obvious if you take a second to look at it form his perspective.
Tampa Bay is offering him a longer deal for more guaranteed money than Belichick would ever give him, to play with better offensive players, and for a coach who will actually act like he wants him to be there.
That's it.
BUH BUH BELICHICK WINS CHAMPIONSHIPS AND ARIANS DOESN'
[email protected][email protected][email protected]#1!!!
Sure, if I was looking 3-5 years ahead I'd bet on Belichick building another championship roster over Arians. But Brady's window is like 1-2 years ahead, and the team is trying to rebound from a 2019 where the offensive roster stunk on ice, and they didn't have the cap space for any quick fixes.
Staying with Belichick is a better play in the long term - but Brady has no long term, because in the long term he's retired.