While most people would feel the need to point to steroid use, I would go another route. Just like today's stars have to show more athleticism and risk taking, you can go back era by era and see the evolution of moves/matches.
Rarely was a "high risk maneuver" used in the 50s-early 70s. There was next to no top rope double axe handle smashes and certainly no atomic elbow drops. Rarely were there anything more than body slams and traditional clotheslines.
Go to the 80s and the high flyer was starting to evolve, which is harder to control both your body, the impact, and hit sell compared to the earlier era. With that increase in risk, one can surmise an increase in concussions, er sorry, "getting your bell rung." You also have to realize this came with taking more bumps and being harder on the body, both the hitter and the seller.
Go to the 90s/2000s, you have folks who's sole role became "extreme rules" by jumping off higher and higher platforms and onto ladders, tables, and chairs. Again, more risk to the body, most likely more concussions, and certainly more pain killer addictions to keep going.
All-in-all, I'd say the evolution of the business had increased the damage to the body over time. Look at some of the greatest performers still alive from the 80s and they are a shell of themselves due to head trauma and pain pill use to get through the day. I feel it will be worse for those from the 90s/early 2000s.