I hate targeted ads as much as anyone, and I don't buy the ad tech bs that they are vital to human survival, but I think the GP is a bit naive to think that ads have no effect on them.
There's long-standing research that people unconsciously prefer familiar things, so exposure to ads is enough even if you don't remember it.
https://en.wikipedia.org/wiki/Mere-exposure_effect
Of course if you would have never ever bought something, an ad is unlikely to make you change your mind. Same if you do thorough research on what to buy, you're probably not swayed by familiarity. But claiming that they have no effect is a bit naive.
EDIT: I guess GP never claimed that they have no effect, but what I'm trying to say is that ads probably have a bigger effect in people in general, and even to you personally whoever is reading this, than you'd think. Targeted ads though? I hope we can live without them.
I feel it quite aptly applies to advertisement industry as well. Massive amounts of complexity and red tape over getting people to buy things
maybe, but that seems so untraceable that it wouldn't be worth the effort to even think about. but, ad people definitely think differently than I do, so who knows what they think about every day.
As for how impactful the ads are - you don't need a study. Simply run an ad and you will know how many people click your ad and how many then follow up with purchasing. That's where terms like cost per acquisition (CPA), customer acquisition cost (CAC), cost per click (CPC), Cost Per Thousand (CPM) etc come into place.
https://www.investopedia.com/terms/c/cpm.asp
It's not one purchase per billion ads. You can look up google Adwords cost per acquisition by industry (WARNING - might be blocked by ad blocker):
https://www.wordstream.com/blog/ws/2016/02/29/google-adwords...
Cost per acquisition on the app stores is much cheaper too.