I really don't care for arguments like this, and I'll explain why. Don Shula coached for 33 years, and for almost all of those years his team was led by a Hall of Fame quarterback. But he managed just two titles in those 33 years, with basically the same roster in two consecutive seasons. He had Dan Marino 17 years and managed one conference title and one blowout Super Bowl loss. So he must be a pretty lousy coach, huh?
Less trollishly: Through the Woodstrock year (1982) Miami fans always felt like they had the best-coached team in football. The Dolphins never made mistakes, they never beat themselves, they always executed. But after that...for the rest of Shula's coaching career, they were exciting, and generally good, but rarely great, and they made mistakes and sometimes beat themselves. And their record reflects that. Similarly: Chuck Noll won four Super Bowls in six years with mostly the same roster. But after that, for the rest of his career, the Steelers were just OK. Vince Lombardi won five titles in seven years with a lot of the same stars. The Packers were clearly in decline his last year, and after he left they fell apart. We'll never know, but I think they would have fallen apart even if he had stayed.
From which I conclude it's really hard to really excel as a coach for a long time. Belichick is now 71, and maybe he's done. But you can't rate him on his performance over the past few years, any more than you can rate Shula on the '93-'95 seasons or George Halas on the late-60s/early 70s Bears.
The argument for Belichick's greatness is pretty simple: in a free agent era, with constant roster turnover, he excelled for 20 years straight. The Patriots were always good and usually great. Nine Super Bowls, six titles, seventeen playoff appearances. Yes, he always had the same quarterback, but otherwise he built multiple completely different but great teams over two full decades. Shula couldn't do that, Noll couldn't do that, and most coaches don't last long enough to even try. Landry did it, but not at the same level of success (two titles and lots of heartbreak). Andy Reid has been really good with two different teams but he still has just two titles, both with the same QB.
When the list of all-time greatest NFL coaches is drawn up, Bill Belichick is going to be on it.