the danger of head shots with a chair are not the occasional severe concussions, it repetitive minor concussions. "they did it before" is true, and that led to Chris Benoit, there is a lot of study happening with sports medicine that is leaning towards consistent repetitive concussions ( usually small ones that go almost unnoticed) having a cumulative effect. the effect of this can be as minor as a little memory loss, to Alzheimer like symptoms, to a full blown psychotic break ( as in the case of Benoit). in the field of professional wrestling there is no way to avoid these minor concussions, every time someone gets thrown to the mat there is a chance of it happening, every head butt, every boot to the face, puts them at risk for another concussion.
before the days of modern tv cameras it was easier to fake a hit, less force was needed to make it look real. with a chair there is even less ability to fake it, as even light force gets multiplied by the extension from the arm the chair has, and the weight of the chair. a well placed chair shot to the head done correctly absorbed correctly is still LIKELY to cause a minor concussion, they just aren't a safe move to use, Title shots less so, as the is no extension out from the end of the arm, all you lose from a normal shot is the cushioning of the hand and arm flexing. yes it's professional wrestling not Ballet, but personally pg or no pg i'd rather have stars retire with cognitive function intact then get my jollys off because chair shots to the back are " lame".