The major point is not so much whether your browser could block ads - your point regarding the browser ultimately having to render each element is true. The problem is that if the web server gets a request from an unattested browser (such as an old version, or one that has an ad blocker installed), it will refuse to serve any content, not just ads.
Regular people will inevitably get frustrated and we end up in scenarios like “<x browser>is bad, it doesn’t work with <y site>” because of this proposal, and more and more people end up switching until you have to use a compliant (Chromium-based) browser to do anything at all on the internet, and Google’s strangehold on web standards solidifies even further.