If so, the tail end of his tenure has been pretty bad. But there can be no doubt he's one of the all-time great NFL coaches, at least in the top 3-4.
It's true Brady played an enormous role in his success. But to those who say he deserves all the credit, I would point out that the Pats also had good to great defense and special teams all through the Brady-Belichick era (and even beyond).
They could not have won 6 Super Bowls without it. And Brady didn't play a lot of D and special teams. Also worth noting that Belichick went 11-5 with Matt Cassel at QB, when Brady missed virtually the entire 2008 season due to injury.
Belichick could win without Brady when he still had topnotch people at other offensive skill positions. He could not do so with both a mediocre QB AND mediocrity in most other parts of the offense. That mediocrity, of course, was in large part the result of Belichick's mistakes as a GM. But other great coaches couldn't win with such mediocrity either.
Despite some weaknesses, it's clear 20 years of incredible greatness outweighs three tail-end years of mediocrity, and one total clusterfuck (this year). If you'd asked any Pats fan in 2000 whether they would take that deal for the next 24 years, we would all have said yes.
In sum, Belichick was an all-time great, and deserves the eternal gratitude of Boston fans everywhere. Patriots Nation salutes him!