Well honestly about half decade I cam to the conclusion that it's only a matter of time. A time when Browsers become OS. I still think that way, but that doesn't mean there aren't hurdles to overcome.
6gb games are still to large to play through a web browser for heavy reasons. Lack of file storage structure, download times are pretty hard knocks against the technology. Clearing a cache will erase the game. Then if you go into downloadable Webapps, then the technology is almost an environment unto it's own.
browsers also aren't exactly friendly towards resoruce sharing. Security is heavy handed. nope, while graphical fidelity is going along nicely there are many other factors that just don't work out so well.
but who knows what the future holds and how browsers may change :)