In a world where users will willingly enter malicious code into their computers if they believe it will do something they want[1], can there really be a better way?
When it comes right down to it, you can't fully protect people from themselves. Even in 'meat space', which the general population is presumably experienced with, people talk others into doing things that they should not all the time. Anything from social engineering to bog-standard scam artists masquerading as door-to-door salesmen.
But in 'meat space', it is way harder and more expensive to do evil against large numbers of people. For example, phishing as done electronically (throw a very, very wide net, and hope) doesn't make economical sense if one had to do it manually.
Also, if, for example, cars and airplanes and banks and nuclear submarines would accept executable code as input, some people would do damage on a gargantuan scale.
Clearly, being liberal in what you accept must end somewhere. I argue that it should end very, very soon. Even innocuous things such as "let's allow everybody to read the subject of everyone's mail messages", if available at scale and cheaply, would entice criminal behavior, for example by those mining them for information that you are away from home.
Does anybody know how RMS thinks about passwords nowadays?
I'm not saying that scams in "cyberspace" don't present a greater threat than scams in "meatspace". I'm just pointing out that if you cannot protect people from themselves in "meatspace", then doing it in "cyberspace" is futile. You can fight it and cut back on it, but you will never actually win that fight. Technological problems to what is ultimately a sociological problem only go so far, and we should be careful to not obsess over them to a fault.
And yet, in "meatspace", chainsaws come with more safety mechanisms than butter knives because the damage they can do is so much larger. Yes, we can't win that fight; people will die from chainsaw accidents, but I disagree that we shouldn't be more vigilant about chainsaws than about butter knives.
This seems like exactly the problem the parent post is complaining about, though: the people in the limited group the parent talks about aren't the people being tricked by selfxss, it's the people who don't have the technical knowledge to understand what the developer console does and why pasting in random JS might be a bad idea. So the phenomenon of selfxss reinforces the point.
Perhaps modern operating systems (or hardware?) need two modes - "Safe mode", where everything is sanitised, checked, limited and Secure Boot-style verified, and "Open mode" where it's not; where experts and enthusiasts can work without limit and without DRM.