> The historical answer does the same, as browsers were explicitly designated User Agents, as their whole purpose is to act in the name of the user, and fulfill whatever the user wishes to, which also clearly places the will of the user over everything a website may wish to.

The browser is a User Agent, I agree. This doesn't mean they can predict all user hostile behavior and fix it before it ever happens. Browsers often fix bad behaving websites over time. Recently there's been a lot of effort to fix pages that cause scrolling jank when they load ads in the background.

There’s been countless cases brought against ad blockers, and all that were about ad blocking, were decided favourable for the blockers.

Additionally, there’s a precedent that users want to be able to control what a site does, and are willing to take extra steps to achieve this, so a browser should implement such functionality ideally in the first place.