That would be a bit of causation/correlation issue.
Sports and society in general are way less violent now than the recent past, i.e. it would be possible for stars player to have been injured more in the past even if enforcer did help (which would be really hard to know, comparing star players playing at the same time with an enforcer on their line versus those who did not had one could be a way to look at, but it would be a really small sample size)
There is a lot of theory around, but lead free gasoline and environment could be one (violence did rise has lead level went up and went down has they went down and the diminution of violence did match quite well geographically in that way)