Why Drew Brees accomplishment of eclipsing Dan Marino's most passing yards in a season doesn't really matter.
Marino did it at a time when QB's weren't as protected as they are now. Heck another player, Tom Brady beat Marino's mark as well and Matthew Stafford missed it by 47 yards.
Even if you wanted to argue "compared to his peers" Drew Brees is better, the numbers below just don't support it. In 1984 Marino threw for 5,084 yards, the next closest was 470 yards away and 3rd place was 1,040 away.
In 2011 Tom Brady was behind Drew Brees by 241 yards then you have to go all the way to 7th on the list, Tony Romo before you get 1,000+ yards between he and Drew Brees (1,292).
I'll take throwing for 5,084 in 1984 NFL Rules over 5,476 in 2011 NFL Rules any day. Much bigger accomplishment. Who Dat?