[0]: http://webassembly.org/docs/faq/#is-webassembly-trying-to-re...
It's not very hard to imagine, especially in an enteprise environment, running a browser 15-20 years from now and that browser loading the equivalent of the JVM, .NET CLR, Ruby VM, etc., on top of WASM :)
AFAIK it never went anywhere, but maybe building an entirely new OS/Browser based around WebIDL seemed less insane 10 years ago.
First of all browsers are committed to backwards compatibility.
Secondly, there's huge amounts of Javascript written right now, nobody's going to throw away billions of dollars worth of investments. People complain about Cobol written in the 60's, when the programmers counted in the thousands. Javacript today is written by millions of programmers.
And thirdly, Javascript evolves, as do browsers.