Don't kid yourself. Bruschi and Faulk were kept on because they're tough to replace and could still provide more even right at the end of their runs than most of the players you could bring in to replace them. They might not have been highlight players but were better than most of the league's roleplayers at their position and they were fine with being paid accordingly, that's why they were kept.
Brady has shown to be fine to be paid like a player in decline, he's accepted far less than he's worth (is there a cap on what a GOAT QB is worth?) and that makes him tough to get rid of, even above and beyond still being elite at his position. But there is no such thing as a roleplaying third string quarterback, you're either the starter or you're not, so the equation is definitely different.
If we have Brady playing crap at quarterback that hurts us a whole HELL of a lot more than having Bruschi as a 4th string linebacker doing his job as best he can and sharing the benefit of his experience with the up and comers. Quarterback isn't a position you let a guy hang onto to the better end unless you either have no choice or are convinced he can rally. One of the two. The first doesn't apply in this situation, if there was a possible opening at the position and no Garoppolo every single out of work quarterback in the league would be sending Belichick love notes. And the second would be a mistake if you're good enough to contend, and BB doesn't make too many mistakes.
There is no one who has a better feel for the situation than Belichick. If he's fine holding pat I'll take that as gospel. If he makes a move I'm instantly fine with it. But if you start ruling things out, you're just limiting yourself, and BB is smarter than that. Don't rule out any possibility. Including the possibility of BB dumping Brady a year or two before TB12 is ready to go if he thinks that's the best thing for the franchise.