You're over estimating the concussion/injuries. There are many people willing to risk injury for millions of dollars, as well as love of the game.
Viewership decline is not due to injuries. Well, I say that, it's a small factor definitely though not necessarily for moral reasons, but more because it removes key players which reduces interest in the game. However, there are many other reasons that IMO are a bigger factor. The primary reason is alternate viewing platforms. People are sick of the commercials, and thus, they are switching to alternate viewing options like Yahoo, Twitter, fantasy channels, RedZone for example - one of their dumbest moves yet. Why sit through 3.5 hours full of commercials when you can watch all the major plays etc... commercial free in 15-20 minutes? There's also some that are getting fed up with the ridiculous penalties. Then there's over-saturation: football used to be Sunday with a strong Monday night game. It had a feeling of special exclusivity to it. Now you have Sunday, Monday, Thursday night and some Saturday nights too. Then add in all the recap shows etc... You no longer need to watch the game to follow the league. Watch your team play, get caught up later. Match ups this year were also poorer, and the networks did a crappy job scheduling - they've skipped over showing some of the most important games, only to show a meaningless game. So, people don't bother watching. Also, there's definitely a loss in franchise quarterbacks - people aren't interested in watching two mediocre quarterbacks duke it out. Then of course, speaking strictly this year, viewership rates were down this fall due to the election debacle. Even my husband, a die-hard fan, watched the debate instead.
So, there's not necessarily one simple reason why NFL viewership is down, but if I had to pick one for the biggest, I would say it's the networks themselves with over-saturation and too many commercials. But - you have to pay for those big salaries somehow, so it's a catch 22. So maybe it just all comes back to money and greed?