> there's no solid proof

I think modern incrementality studies give solid proof of the value of an ad on the basis of a good model of how the world works. Of course, models can be wrong - it could turn out that solipsism is true, physics is false, and the world outside of your own mind is a figment of your imagination!

That the world is complex and models are inherently wrong does not mean that nothing of value to businesses was lost with ATT.