I think you're assuming a lot here. Many countries around the world play rugby, from which American Football is derived. The only use of helmets in that sport is for covering players' ears if they stick out and are vulnerable to getting torn. It's just as aggressive as AF, but (preferably) played at something closer to the speed of soccer, without all the committee meetings and personnel changes (which everyone outside the US finds mystifying and dull). Obviously the players are not as big or colliding with such force as a result, but the game isn't any less popular for that; and happily, it's not highly correlated with brain damage (unless you count rugby fans' notorious appetite for beer). In short, you can have sporting machismo for fun and profit without necessarily needing all the players to look like the Hulk.
Football is America's premier sport, and the Super Bowl is America's most prestigious sporting event.
The super bowl is certainly a big deal because it all comes down to one match, but I think it might be interesting to aggregate the interest and profits in the World Series of baseball (best of seven games between the top two teams), rather than compare a single baseball game with a single football game. Baseball is arguably a more successful export than American football, and has greater long-term growth potential. Although NFL is about 2x more profitable, and attracts bigger crowds, there are 10 times more baseball games than football games per season, so overall attendance at baseball stadia is actually 5x higher than at football stadia (http://www.plunkettresearch.com/Industries/Sports/SportsStat...).
Boxers and MMA fighters knowingly take up a sport where their retired legendary heroes can barely form a coherent sentence after age 50. They know going in they're going to get their brain damaged and they do it anyway, and boxing's not half as prestigious as football here.
But boxing used to be hugely popular; our greater understanding of the long-term damage which results, and exactly how it results, are IMHO a significant reason for the decline in popularity. When I was a kid Muhammad Ali was a megastar; he may not have made as much money in $ terms as someone like Tiger Woods today, but he was arguably the first globally famous sporting figure. The heavyweight boxing championship was considered a must-see event, like the premier Olympic events or a world cup final. Now, not so much. MMA or ultimate fighting or whatever it's called is well marketed and profitable, but it doesn't have anything like the prestige that boxing used to do. If you stopped 10 people on the street I doubt that more than 1 could tell you who the MMA champion is, whereas anyone over 35 could name some former heavyweight boxing champs.
I'd bet $100 that American football will go the same way over the next 40 years. Not because the NFL or even a single NFL team will decide to adopt a more delicate style of play, but because parents are not going to be enthused about the risk of their kids getting brain damage in high school or college while they're aiming for the NFL. Once a player goes pro, football pays so much that they'll probably be able to cover their future medical expenses. But brain injury to a non-pro player has the potential to be just as expensive, and if the injured player never gets near the pro salary then the cost lands back on their family or possibly the taxpayer.
Lately the NFL has been marketing itself heavily to women because they consider the male sports audience to be saturated at this point. I think greater female interest in the sport will boost it in the short term, but as more of these fans become parents there will be a correspondingly greater brake on new entrants to the sport. Personally, I think NFL has already peaked, and can only go downhill from here.