True, there are ad-ons to address such browser deficiencies. The question is why browsers do not have these capabilities natively. Clearly, a large number of users want them.

Note that the 20% in your estimate are quite significant.

As for "breaking websites", perhaps in the evolution of a web to be faster, those "certain websites" with too many third-party dependencies should be selected against, not for.

Another interesting question is whether something like HTTP/2 developed by an advertising company encourages the inclusion of more third-party resources, perhaps in the form of advertising and tracking, or does it encourage less, in the form of smaller, faster, safer websites? It appears to be overkill for the later.