It won't happen. It's an impossibility. People will always say Manning was better than Brady.
ESPN backed their horse on this one long, long ago, and that's just the way it's going to be.
Here's a better question. Why does it matter to you, that non-Patriots fans feel a certain way about a certain player? Seriously.
I would say that at least 80% of the Patriots fanbase feels very strongly the sporting world should accept Brady as the #1 QB of all time. To me it's pretty self-evident why that's the case. It's a natural offshoot of rooting for sports teams in general. We want everyone to regard our team as the best, and that's why they play the games, because the best team can objectively be determined through competition.
But when we get a QB on Brady's level, it goes beyond that. He can potentially be considered the best player in the history of America's favorite sport. That is as important to many people (including me) as which team is the best in any given year. Who would have thought we'd be saying that when Bledsoe went down against the Jets. I live in the Bay Area and you can bet my friends who are 49ers fans are rooting against Brady purely because they still want Montana to be #1. Unfortunately, that can't be determined objectively, so instead we build a case in his favor and back it up with statistics and reasoning. We set set parameters for the discussion, like Superbowl wins being important criteria, and then have to argue about whether those parameters are relevant given that it is a team sport.
There is a reason Patriots fans reacted with such a backlash to DeflateGate, and it wasn't just that we were penalized for it and it hurt our ability to compete on the field. We did all the scientific research and read the appeal transcripts and learned about the legal appeal process because we DeflateGate damaged Brady's legacy and hurt our demand that the world accept him as the greatest ever. Just look at the visceral reactions at everyone on this board on a daily basis to Casserly saying something dumb or someone else praising Aaron Rodgers and it's clear that almost everyone cares about Brady's perception nationally.
It's hard because the sports media, and opinions of uninformed fans, are generally so stupid and unaware of the facts, that you might consider it to be a huge waste of time. I sometimes try to ignore it because it makes me so furious when people are dismissing Brady as the greatest because they still think 11 of 12 balls were 2.0 PSI under. I try to just be happy and grateful that I have been lucky enough to watch this guy for 15 years and experienced the joy that we all felt when he took us to the top of the mountain, and know for myself that he's the greatest ever. That should be enough, along with never letting any hater take that away from me. But to live in that state of zen is exceedingly difficult. If you can listen to some guy at the bar or your friend or whoever disparage Brady for cheating or being a system QB or whatever other reason, and not argue with that person (because who cares what a non-Patriots fan thinks, right?) then you are a better man than I.