Come on, many people are looking down at JavaScript for a reason - if the new standard allows proper multithreading, it suddenly gets closer to "usable" language as defined by many. But if they had a choice of another language running on top of Web Assembly (or whatever you name the web's VM), I am not sure they would even consider learning JS despite it being finally on par with their favorite language.
JS won't be entirely obsolete. You won't be able to access the DOM from WebAssembly. It will be way easier to make way more performant web apps though.