I really don't buy that. I know it's what is sold all around the media but I don't buy it. Professional rugby league players here, who play 80 minute games without protective equipment, play a 25 week season followed by 6 weeks of playoffs. In the off-season they play in international games. These guys also play the full 80 minutes week-in-week-out as opposed to limiting themselves to defense or offense. I'm not saying that these guys are 'harder' than NFL players or anything, and their body compositions and fitness levels are entirely different, but it shows that professional athletes in a full-contact sport can last that long.
It also shows that athletes, being professionals, can adapt their bodies accordingly. Therefore, if the season increased, training would change to fit the new season length. It's about adapting the body.