WebAssembly’s FAQ page agrees:
“WebAssembly minimizes costs by having a design that allows (though not requires) a browser to implement WebAssembly inside its existing JavaScript engine (thereby reusing the JavaScript engine’s existing compiler backend, ES6 module loading frontend, security sandboxing mechanisms and other supporting VM components). Thus, in cost, WebAssembly should be comparable to a big new JavaScript feature, not a fundamental extension to the browser model.”
Both cannot replace each other because both have unique pros and cons. But, ultimately, it depends on the web developer's choice.