Reporting a conversion rate is not the same as reporting that _you_ as a particular user converted.

For advertising to remain a viable way to support portions of the web, advertisers need to understand the effectiveness of their programs and marketplaces need to understand the exposure.

Specifications like PCM are competing mostly around how to provide that, although there are also efforts to sandbox local personas to make ad presentation decisions on-device. Personally I'm not so much against those efforts, as it doesn't seem like a betrayal for my web browser to try to select information it thinks I'd more likely care about - as long as I can also choose how/if it does so.