Considering that most websites are optimized to just barely run on what the developers are targeting, letting them target the "state of the art" instead of what came installed on grandmas machine is been a huge negative for website efficiency and thus average user experience, not to mention global resource waste.
Yeah, that's where you lose me. The people who want to guarantee pixel-perfect rendering of web sites tend to be the same people who feel entitled to run their code on my machine as part of delivering their site, who prioritize their brand identity over my accessibility settings, who want to build their complex web app UI with no respect for the conventions of my chosen operating system. My browser should not be an ally to them; it should be taking my side in those conflicts.
Can you imagine going to see a doctor, and in the middle of your appointment, the doctor drags you into the operating room to automatically update your body?
That's roughly how I feel about automatic updates. If you apply the concept of automatic updates to any industry outside of software, it would very easily be illegal. But strangely, this concept is considered legal when it comes to software.