Advertisers will just demand that web sites using their ads install gateway software to make their cookies appear local, along with software that will block displaying content if ads are blocked.
Which the browser makers could hopefully defeat by pretending to accept them and/or accepting them for the duration of browsing that site and then delete them, which would limit their snoop value.
Unfortunately you are right. That will take a while to become the norm, but it’s already happening some.
The reverse will also happen in response, where ads (if they can be identified as ads) will be accessed but not actually displayed. It will keep the display looking clean, but will tax connections with ad traffic anyway, and will pass through tracking information to the advertisers. They won’t get any clicks though!