Worst case: monetized in the non-free way. Which might eventually turn out to be the best case.
Less dramatical outcome: roughly the same amount of advertising, roughly the same amount of ad spend/income, just not quite as targeted as it is now. Maybe you are too young to remember, but there used to be a web where not every website showed exactly the same ads to a given user. Ads where targeted more by context/content of the site than by browser history. That era made a certain company very rich that happened to be quite good at inferring context/content of websites using crawler technology.
Most likely outcome: not really any noticeable change, ad mediators will find a subset of tracking data that is sufficient to keep up their current level of targeting creepiness that isn't in judicable contradiction with the GDPR.
Less dramatical outcome: roughly the same amount of advertising, roughly the same amount of ad spend/income, just not quite as targeted as it is now. Maybe you are too young to remember, but there used to be a web where not every website showed exactly the same ads to a given user. Ads where targeted more by context/content of the site than by browser history. That era made a certain company very rich that happened to be quite good at inferring context/content of websites using crawler technology.
Most likely outcome: not really any noticeable change, ad mediators will find a subset of tracking data that is sufficient to keep up their current level of targeting creepiness that isn't in judicable contradiction with the GDPR.