You're not being honest with yourself. It's all about comfort for the fans. And the age argument exists, evidenced by the continuous posts in this thread alone from older fans who don't want to sit in the elements anymore.
This is not about the game or the players, for multiple reasons, but the mere truth is that this game has always been played outdoors where we've witnessed the greatest players thrive and break records. We just witnessed the most perfect offensive game in NFL history in an outdoor stadium. We've recorded the loudest crowd decibel levels in outdoor stadiums. And these events will continue to take place in outdoor stadiums.