Companies like Google, Meta, and Amazon have massive first-party user data from their logged-in ecosystems, so advertisers would simply redirect their budgets there.
If the GDPR were fully enforced to the strictest interpretation used by various information commissioners, meta would have its revenue side almost entirely wiped out, because targeted advertising is not strictly necessary for all of the functionality that the user wants from Meta, and the content-targeted "suggestions" would also be a separate function to the communication with friends and family that attracts and retains users, and so the use of personal information would need separate consent too.
For google, properly applying the GDPR and banning tracking would effectively split the data provided for search, mail, data storage, etc. from the advertising functionality, so they'd effectively just be hosting Adwords on their properties.
If, say, the EU completely banned tracking for advertising, the open web would collapse almost overnight. Ad revenue would crater because behaviorally targeted ads sell for several times more than generic contextual ones. Small and mid-sized publishers that rely on that revenue would struggle to survive, leading to site shutdowns, mass consolidation and paywalls across what used to be a free internet.
The independent forums, blogs, and wikis that make the web rich and weird would shrink dramatically.
The open web was a lot healthier before the rise of tracked advertising, especially considering the much higher cost of equipment and bandwidth back then and the lower number of users. Wanting to insert tracked advertising was also one of the drivers for web-based platforms replacing News, IRC, and so on, which was definitely a step in the wrong direction.
Newspapers, already on life support, would see digital ad margins collapse,
The big complaint from the dead-tree newspaper industry was that tracking-based advertising made their contextual and locational advertising uncompetitive. Banning tracking and abuse of user data would effectively wipe out all the advantages online advertising has over traditional advertising, apart from duplication and distribution costs.
Hobbyist sites would be hit hardest. Most rely on cheap targeted ads and affiliate links just to cover hosting.
Really? most of the ones I use rely on contextual advertising or (for forums etc.) selling status markers, because there's no money in generic ads without a huge volume.