The Patriots have opened 21 games with someone else in the position since the start of 2001: two before Brady took over when Drew Bledsoe was injured, 15 after he hurt his knee in 2008 and Matt Cassell took over, four when he was suspended in 2016 for Deflategate and Jimmy Garoppolo took over. New England won 13 of those, lost eight, a .619 winning percentage. Not bad for working with backup QBs, even guys as talented as those three.Of course, the greatest ingredient in whether Belichick can succeed without Brady is who is installed as his replacement. Starting quarterbacks are not as easy to find as he, when required, has made it seem. And the most important element of Brady’s success is the circumstance he chooses to enter. He needs a high-level offensive line and some capable (at least) route-running receivers. They both could fail badly in this department.We will learn in the coming season or seasons how well they do separately, whether Instant Karma is going to get either one. Or both. The six Lombardi Trophies won by the New England Patriots are on display at the team’s hall of fame and museum. Nine of the 27 players honored by the Pats played at least some role in claiming those Super Bowl titles. And yet we all know this dynasty is primarily the work of two men.Bill Belichick and Tom Brady are the most successful tandem inside a broader concern since John Lennon and Paul McCartney were forming the core of The Beatles. Those guys eventually split up, though, and so it is with the modern Patriots. Belichick will stay in New England and remain the Pats’ head coach. Brady will go elsewhere, and not merely to sit beside Giselle on some (private) beach.If they could not remain together, this is the wise choice for the Patriots.MORE: Why did Tom Brady leave the Patriots?It is possible that Brady, who will be 43 when his next season begins, has several more years of excellent football in his right arm. It is possible Belichick, who soon will turn 68, shortly may decide he has called enough blitzes and cut enough special teamers that he would prefer to spend his remaining years with his girlfriend, Linda Holliday. Either way, each man is closer to the end of his career than to the beginning.Not only does Belichick’s future appear more extensive, though, he also has been the primary reason for all that excellence. Brady has been a spectacular quarterback, the ideal player to execute his coach’s vision on offense, but there is so much more to what has made the Patriots great than the number of points they score.How do we quantify this? How can we measure one relative to the other, because they have been together for so long and elevated each other so much?Lennon and McCartney shared writing credit on all their Beatles songs, but it was often obvious which one wrote most or all of a particular tune. “Blackbird” was pure McCartney. “Strawberry Fields Forever” had Lennon written all over it. You could even tell which wrote what part of “A Day in the Life.”With the Pats, the separation is even more clear. Belichick has been in control of much of the operation, even before Brady arrived. In fact, he quite obviously brought Brady into the squad. And although a head coach such as Belichick does not have singular control over every defensive or offensive call, or every piece of advice imparted by his position coaches or coordinators, it is his operation.The Patriots finished first in the NFL in scoring defense three times since the dynasty was launched in 2001. They finished first in scoring offense four times. They ranked higher in offense 12 times, in defense five times, and twice they were equal. Even though Belichick’s background is as a defensive coach, the offense has been the primary driver of success.MORE: Who will be Patriots’ new QB? Exploring the optionsBrady essentially is 0-0 without Belichick as his coach. He entered the league in 2000, became the starter in 2001 and has compiled a record of 219-64 as starting QB.