Those footballs that they used to head 40 years ago were so heavy and also even worse when wet, which is why are seeing the correlation now, as players like Astle's brains are examined. Sure it can't be good for any footballer now to constantly head the ball and more research needs to be done on the modern ball and effects on the brain.
I was inclined to agree with the argument on ball weight.
But then realised energy = 0.5* mass * velocity**2
So halving the mass only works if the ball speed has not risen by more than sqrt(2).
So it may not be quite that easy.