Everyone has always known that it's detrimental to the body. The thing that's new is the science that shows what it does to the brain. This is a sport that apparently causes widespread dementia in people in their 40s and 50s. Their minds are gone, their behavior is bizarre, their relationships are destroyed, and sometimes they kill themselves. This isn't just "well, you might get hurt if you play and everyone understands that." It's "well, you might up being Chris Benoit" (whose brain showed exactly the same kind of damage that many NFL players' brains have).
If you don't understand how that moves beyond ordinary risk and into the territory of something that civilized society might find unacceptable, then there's really no point discussing it with you.
I don't think anyone would disagree that this is something that grown men who make money can choose to do. Just like boxing. What about high school? What about unpaid college players? How's the NFL going to survive when its feeder system starts drying up?