Well, this has been a fun discussion. A salient point that I haven't mentioned but that actually plays a big role is attribution. Many advertisers run ad campaigns not to get a clickthrough to their site, but to just keep their product 'top of mind', so that next time you go buy a car you buy a BMW. So from that point of view, if you see an ad and later go and sign up for a BMW test-drive, BMW would attribute that test drive to the ad that you saw. If you can't track attribution, it becomes really hard to figure out where you should be advertising in the first place. To everyone saying "use contextual advertising" - how do you know which contexts produce better results if you can't measure performance?

This is particularly relevant to mobile apps, because if you show the user an ad, they are extremely unlikely to switch contexts to go and actually click on it. If you can track users from the app to the purchasing site, then you can say "hey, I have a really valuable audience - you should pay me hella money to show them ads". This has been less GDPR and more Apple, but the result is the same - it makes ads generically less valuable.

And that is why I'm fundamentally pro ad-tech. I don't have any direct monetary interest in it, but I do want the digital economy to be growing and efficient, and in an ideal world decentralized beyond the 4-5 large platforms. People spending money online is GOOD. It's good for businesses, it's good for the people (under a rational agent model at least ;)), and it's good for me as a software engineer who wants to keep getting paid silicon-valley salaries.