Furthering the pedantry, even assuming monotonically decreasing ROI as ad spending increases, as long as ROI of the first dollar was positive, there would be so

Furthering the pedantry, even assuming monotonically decreasing ROI as ad spending increases, as long as ROI of the first dollar was positive, there would be some optimum between zero and current levels. But my original statement still holds, zero would be preferable to current levels of spending for those brands.