That's a red herring. Those of you bringing up the changes from 12 to 14 and from 14 to 16 are either forgetting or leaving out a few factors that were in play then but are not now.
1) there was no free agency during those years. Meaning a player was forced to stay with the same team until that team didn't want him anymore. That sounds terrible, and it is in a free market society, but it was also beneficial for the player in a way in that teams took time, and resources, to develop players without fear of losing him 4 or 5 years into their careers. Since there was some stability in terms of movement, players lasted longer with teams, which allowed them to master their systems and extend their careers. There were no " specialty LB or Linemen " in those days. Most, if not all, played 3 downs, and alot of them play special teams as well.
2) No salary cap, meaning teams didn't have to cut players because of salaries.
3) probably the most important of them all: NO YEAR 'ROUND FOOTBALL. There were no " QB schools " or " OTA's ", other than a workout camp for draftees after the draft and 1 mini-camp about a month or so before training camp. Players worked out, often, on their own at the teams' facilities, but would use training camp for what it was originially intended for: training.
4) Although there were still 4 pre-season games, teams were allowed to have as many players ( Cowboys ususally carried over 100 ) as they wanted in training camp.
Todays NFL player ( on average ) is probably 2 times, 3 times, faster, twice as strong, bigger, more sophisticated, better fed, more talented and in alot better shape than players in the past, yet they're not lasting nearly as long. Saying that there's no " magical line " that determine the limit of a human body goes against all evidence that's out there.
Everything has a limit and nature has it's way of enforcing that limit. Be it by injuries becoming more common, or by careers getting shorter and shorter as time goes by, but it will not change.
