Webassembly is specifically not designed to replace Javascript. It says so right in the documentation.

See also this: https://www.quora.com/Will-WebAssembly-make-JavaScript-skills-more-or-less-valuable-in-the-future/answer/Quildreen-Motta?srid=dafu&share=7cf60d50

I do agree, however, that the future of pure JS will be as a target for transpilers .