Having made things the way they are, if they try to change the rule now, I forsee a gaggle of lawyers waiting to sue on behalf of someone who was playing after the rule change (or, more likely, on behalf of his insurance) and ended up with post-concussion syndrome.
"The committee put the rule in place in response to well-founded concerns about player safety, didn't it?"
"The committee then, despite clear evidence that it was having a positive effect on players' tackling technique, relaxed the rule, didn't it?"
This line of thinking does not finish anywhere good for anyone whose rate is under $500 an hour.