For evidence, you can review the research of Dr. Anne McKee of Boston University. One example:
http://judiciary.house.gov/hearings/pdf/McKee091028.pdf
She has personally disected 12 brains from former football players suffering from chronic traumatic encephalopathy, of which 10 were linemen or linebackers. The patients exhibited all the symptoms of Alzheimers or dementia, but their brains are very different, showing no typical indications of Alzheimers brains but a buildup of 'tau protein,' also seen in the disection of boxers brains.
The rate of dementia diagnosis is currently 1 in 1,000 for men aged 30-49. For NFL veterans aged 30-49, that rate is 1 in 53. That's one on every team.
While you are probably right - it will be difficult to conclusively prove a direct linkage between a three-point stance and an individual case of dementia, just as it is similarly difficult to conclusively prove other issues of causation - it will be increasingly difficult for the NFL to ignore the correlation. And, as noted, the three-point stance change would be part of a broader approach to change.