I have not read the paper but I have done extensive ad effectiveness analysis and the one thing most papers ignore is that having your product listed in a store is a given. More often advertising is protecting your product's listing in a store. Ad spending isn't just about boosting your own sales, but those of the category and adjacent categories. If those 80% of brands stopped advertising to prevent $1mn of negative ROI but then were dropped by the store, causing $100mn of negative ROI.