But there's another angle: @acfou estimates fraud at 20%, It could be 40%. Google takes 60% of ad spend, $80B in USA last year. 0.2 x 0.6 x $80B = $9.6B; 0.4 x 0.6 x $80B = $19.2B.
Flip side is malvertising: exploit kit loader hiding in ad, criminals pay for low-priced ad slots, make big bucks via ransomware.
No publisher I know of can afford to cut all programmatic ads. The ads.txt file has zero integrity (typos, DNS spoofing, listed vendors reselling inventory under table). Heavy JS "nanovisor" approach is heavy.
Ad spend may drive sales but marketing is not science, lots of correlation isn't causation trouble. Bad sales? Blame weather!