The sheer number of big corporate backers, and standardization, is what will make it happen. That's really what is different here versus Java applets, Silverlight, NaCl etc. Those all failed because nobody was big enough to single-handedly push something like that onto the ecosystem. Now that they're acting in concert, things are very different.

Everything else is "just engineering". E.g. as far as being complementary to JS, and not being able to replace it - they are already working on access to DOM.