This announcement means JavaScript can no longer rely on "you must target JavaScript to work on the web" as a motivating factor. The set of people who like JavaScript as a language in its own right can keep using it, and will have the advantage of new JavaScript engines and extensions implemented on top of WebAssembly. However, the set of people who would rather write in another language will no longer be constrained by waiting for JavaScript to catch up.
Personally, I think this will be a good thing for both JavaScript and other languages.