It's definitely turning into a trend, starting with Chris Borland a couple years ago. Borland was one of the first rookies to just outright retire unexpectedly, citing health concerns.
Since then more young players have opted to hang em' up. There has to be some kind of real downside to playing certain positions to leave all that money on the table. No talented young player with the capacity to rack in 10+ million garaunteed doing the thing he loves would turn away from that unless his closest advisors and family/friends were worried and convinced them it isn't worth the money.
Look at a guy like Anquan Boldin. Might be the toughest player I have ever watched during my lifetime at the WR position. Dude got his face rearranged and still came back, and came back to be effective for many years. All signs point to him being able to live a long post career life without being disabled.
I believe it's more about the system you play in, the asmount of snaps you take, and what position you play rather than just saying the entire sport is tainted because of risk levels concerning mental damage.
Mike Singletary lit up more guys than I can care to count, and he was involved in a TON of brutal collisions during an era where the rules were still ruthless for the most part. He was a terrible head coach, but it wasn't because he has dementia or is compromised mentally. The guy speaks clearly and with purpose, his faculties are obviously intact.
Other unfortunate players do suffer from CTE and end up being cripple by 50 years old. I think there needs to be more research before we say the NFL is finished because of brain related injury.