[jesstaa]

Proxying increases the cost and effort of tracking. It's more involved than simply adding a bit of JS to a page. Things like the Facebook 'like' button is a tracker that people adding it to their page won't even realise was a tracker and thus would be less likely to put in the effort to proxy that to Facebook.

For sites that make their money from advertising adding proxying would probably be a requirement and be worth the effort, but for most other sites that are trackers by accident it wouldn't be.

Restricting third party cookies won't prevent all tracking but it will increase the cost and reduce the number of sites doing tracking without knowing it.