So in order to accomplish what you're saying, all attesters would have to reject all browsers with extension functionality then, right? And if they really wanted to eliminate ad blocking, those browsers would not even be allowed to run debugging scripts.
I don't see a lot of buy in from users to such a system. The proposal requires the site, the user, and the attestor to comply. I don't see any plans for an overhaul of the entire tech infrastructure.
The worst that can happen as I understand it is a handful of websites will start blocking users who aren't validated per the spec, they'll display a message like "this website only works in BrowserEveryoneHates", and then a competitor will swoop in that works in every browser.
The best that can happen is users will have a little more security from tampered software, advertisers won't lose as much money from bots, among other things as they describe in the spec.
I'm open to changing my mind, but this is just how I understand it so far.