Yes they should. It's the only way the league will change to focus on injury prevention instead of ratings and will continue to provide adequate healthcare services to athletes after their careers are over, as well as offer appropriate support to families of athletes.
You are incorrect in assuming that this was a known risk to players when he started his career, which was in 1990. The research on the long-term brain damage associated with the sport (chronic traumatic encephalopathy or CTE) is very new and still evolving. Serious research into CTE began at Boston University in 2008.
Did players know that concussions happen? Sure. Did players know that concussions would lead to permanent brain injury and mental illness? Did their families know to look out for certain symptoms as signs of permanent cognitive and behavioral change? No.
What will come out in this lawsuit, and others like it, is how much the league knew or should have known about this type of injury. Did the league neglect its duties to protect and inform its players of the risk? Did the league invest adequately in brain injury research and prevention, or did it ignore serious warning signs? Did the league encourage or mandate injured players to get back on the field and play through concussion and if it did, did they know or should they have known the risks associated with that?
Lawsuits like this are a way to ensure that *if* the league bears responsibility, they are made to take that responsibility where it hurts, which is the wallet. If there isn't evidence to support that the league knew or should have known about these kinds of injuries and have protected players like Seau better while he played and after he retired, then the plaintiffs will lose.
At the end of the day, lawsuits and media coverage of these injuries, and the continuing research into brain injury, are making schools and athletic programs at all levels take this much more seriously. I had a student in an SAT class 4 years ago who sustained a concussion on a Friday night in a football game. His practice test score a few days later dropped more than 300 points from the last one taken prior to his injury and he was clearly out of it in class. Yet he was in school and played again the following Friday. If that had happened now, he would have been on athletic and cognitive rest until his symptoms subsided and then would have slowly eased back into school and sports. My oldest son had a concussion at the end of November and was out of school for a week, on academic rest for another week and didn't start back playing hockey until after 3 weeks.