I don't think this is ever going to happen. JavaScript is still and probably always will be the language used to manipulate webpages and interact with the user, it's extremely optimized and a quite fast and effective scripting language.

WebAssembly provides an amazing opportunity to do general computing in webpages more efficiently than with the JavaScript scripting language, but it's not designed to interact directly with the user or documents. Certainly that _could_ change in the future, but that's not one of its design goals and I don't think it would even be particularly effective at it.