Which begs the question, have advertisers ever actually measured effect scientifically, and whether the arms race of acquiring eyeballs for no net position change will collapse.
When John Wanamaker said what he said I think he was talking about print ads for department stores in an era before ad metrics, large scale market research, etc. It was probably true then to be fair, but that doesn't make it useful now, or applicable to all advertising. For example, if I used this quote when I was talking about Internet adverts I'd just be wrong. It's trivial to track the effectiveness of an online ad