First off their is a lot wrong with this. Mark Rypien, Drew Bledsoe, Jimmy G, and Cam Newton all started Super Bowls.
Second off, having a QB who appeared in a Super Bowl is not a prerequisite to having a winning record or winning more than one playoff game. Marvin Lewis has a winning record despite never having a QB who appeared in a Super Bowl over his whole career. Belichick as you mentioned has 3. Sean McDermott has a winning record without ever having a QB who appeared in a Super Bowl and he won 4 playoff games in 6 seasons. Sean McVay had Goff and Matt Stafford (who never won **** his entire career) and got both those underachievers to winning playoff games and appearing in Super Bowls.
I'm sorry but I can't really take these excuses seriously, especially when Bill was building all these teams The only time he had a GM overseeing him was in his first year in Cleveland. ****ing Daboll just won a playoff game with Daniel Jones last year and equaled Belichick's number of playoff wins without Brady with that piece of junk at QB. Mike Vrabel has Tannehill and has a winning record and more playoff wins than Bill does without Brady. Kevin Stefanski who sucked found a way to win a playoff game with Baker Mayfield. d%#*!@t, Pederson had Nick Foles take his team through a playoff run and did more with him in one season than Bill ever did without Brady.
It's just a fact, Bill Belichick, who has had completely control of every roster he's been HC of since 1992, has a losing record and 1 playoff win in 11 full seasons without Tom Brady. That's a big enough sample size to call it what it is. The fact that he's had 4 QB's capable of getting to Super Bowls in that time frame isn't nearly the excuse you think it is. If anything it's an even bigger negative when other people did more without a QB who made the Super Bowl, let alone 4.
Also Matt Cassel was in 2008 not 2007.
I don't even know what you are trying to prove on this.