> Browsers would have to specifically block the extension if they wanted to keep people from using it.

Which Google have done to other extensions many times.

Yes, it's possible—I would say probable—that after introducing their "impartial" ad blocker, whose rules for acceptable ads just so happen to allow all of their ads through, and after enabling it by default on their browser, they will actively block all other ad blockers, on "privacy and security" grounds.