It also comes down to style of play. I don't like the idea of punishing Brady for having a style of play which promotes health and longevity compared to a style of play which is higher risk and wears you out sooner. If Montana played in the current NFL he could have played longer, but not as long as Brady. If Brady played in the 80s, he wouldn't have played as long, but would have played longer than Montana or any other QB for that matter. Also he would have had more healthy playoff runs than Montana with his stacked team. Often the 49ers seasons were derailed by Montana being unable to stay healthy. And that comes down to his style of play as much as the era.
Also, while I know you mentioned nothing about this, but to respond to the idea... can we PLEASE stop comparing coaches like it is some HUGE advantage? The only time coaching is a huge advantage is when it comes to lapses of competence. the difference between a great and good coach in the NFL is rather small, and I think people still underestimate how much Brady meant to the 6 super bowl wins and over estimate how much BB meant. Reid has incredibly innovative playcalling and had a track record of playoff appearances and deep runs before Mahomes ever suited up. Something BB sure as hell didn't have as a head coach. Also Walsh is clearly an all time coach. An innovator on offense. It's called the west coast offense for a reason. To say that BB was some kind of huge advantage over these guys, is an overstatement.
And it isn't like Mahomes or Montana suffered from Bad coaching or an unfair advantage talent wise. Brady took over a team that was 0-2 before he started and 5-11 the year before that. While clearly not as bad as that would suggest, because they had some talent on the defensive side, they were hardly a stacked team. Mahomes walked into a team that went to the playoffs 3 years running and made Alex Smith a 2 time pro bowler. Montana talent was stacked pretty much his entire career with a few exceptions, on both sides of the ball. The idea 'those first 2 super bowls he didn't have the GOAT WR' is silly. First off, he had the GOAT WR for the rest of his run. Not having the GOAT WR doesn't prove you didn't have great offensive talent around you. And unlike Gronk, Rice was almost always healthy, Clarke and Craig were better players on offense than anyone Brady had in that early run, Save 2004 Dillon. But that was only 1 year. This isn't to down play the guys Brady had around him. Or tear down Montana. Montana is the the 2nd best all time and by a comfortable margin IMO.