If the website says to display ads, my browser will properly ignore that. If the website says to prevent the user from switching away from the tab, my browser will properly ignore hat. If the website tries to take over my system and install malware, my browser will properly ignore that. And if the browser tries to break the ability to remember passwords, my browser will properly ignore that too.
Websites consist of code to be interpreted by browsers as they see fit, for the benefit of their users. Those users do not necessarily want exactly the experience the site authors want them to have.
The difference between autocomplete=off and the rest of your examples is that there are actually positive UX use cases for disabling autocomplete on certain inputs (e.g. when you are an admin editing existing users)
Ad blockers have false positives as well. And there's a use case for blocking the user from closing the tab (onbeforeunload), such as prompting them to save/submit what they're working on. But for all of those, the browser is still in control and the question is what provides the most benefit for the user.
So, along the same lines, it may make sense to improve the UI for autocompleting users, or for hinting about the use of the field, to make it easier for sysadmins. But that shouldn't break the more common case of handling sites that just think they're Too Special or Too Important to allow saving login information.
Websites consist of code to be interpreted by browsers as they see fit, for the benefit of their users. Those users do not necessarily want exactly the experience the site authors want them to have.