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to answer your question - everything you listed. We can pretend the computer knows our intention but it doesn't... we can pretend computers are responsible for our safety online but they are not...
and I get that when it comes to google - I get to use their services for "free". But I pay uber and many other companies for their services and I feel I'm entitled to privacy of data they have about me...
So please tell me, how would you design a system where you can't install software that contains a tracker. Any kind of security is only as strong as the weakest link. The OS/browser, if coded "right", can only protect you from unintended infection. if your computer nicely asks you if you want to install this crap and you say yes then it's your fault - not the developers... With dumb user interaction ANY OS can be attacked...
but you do already have "local storage" - they're called cookies. local storage (if it was actually used) provides safer way to store "cookies" (just an example). As a consumer you get a better experience and higher safety. If you're asking why, as a consumer, would you want anything beyond basic HTML then honestly I have no idea what to tell you.
And I mean come on, if I'm at the point where I need a wheelchair on earth... If I can choose I want to go out during a rocket launch (with a bang! KSP does that to you) rather than in a hospital bed.
The thing is - everybody is responsible for their security. We don't need to "go back" - we need to teach users how to be safe. I check my parents computer whenever I come see them. No toolbars, no malware, no viruses - because me and my brother took the time to teach them basics of computer security (and mostly to click "no/cancel" if unsure).