1) Zero-pain installation (no steps, no wait to download)
2) Zero-friction sharing (paste a url to any resource within the app/game)
3) Zero-friction upgrading (happens automatically. depending on architecture, no need to even reload the app)
4) Largest install base of any platform
It's a classic disruptive technology. Yeah the graphics suck. Yeah it's slow. But those things are improving, and will get to the point where they're "good enough" for a big swath of uses. Whereas the four properties above are much more challenging to fit into an Xbox or iOS architecture.
Web links are cool. Just about the only redeeming feature of the web. They have absolutely no connection to JavaScript, HTML5 or any of the other hideous complexity you see in the browser.
Browsers are not "zero friction" upgrading. Behaviour changes from version to version. Using Firefox on Linux I have experienced continued breakages. Again, if you had a simple virtual machine it would be simple to port anywhere, and you wouldn't need constant bug fixes.
Your last point has nothing to do with technology. It is simply the result of vendors successfully selling the technology to people. By doing so they've locked other better approaches out of the market due to network effects.
The remainder of your post is the good old "it'll be good enough" attitude. Except it won't ever be as good as it was because people are reliant on benchmarks that make "freight train" style systems look "fast" for through-put code.