There is a tradeoff between development speed and execution speed. For web apps, development speed is usually the most important of the two and the language ironing away the most technical details tends to be a strong choice.

Roughly speaking, C is further from the metal than assembly (and WASM) for obvious reasons; Java is further from the metal than C (or Rust) due to having garbage collection, and Javascript is slightly further from the metal than Java due to abstracting away threads (the gap was far wider before Java got lambdas). LISP, of course is off the scale due to abstracting away readability ;)

On a serious note, though, I don't see the advantage that Java or Flash or Silverlight would bring to web apps unless they bring greater development speed.

But I do see WASM as an excellent future supplement to JS-based apps, for optimizing time consuming parts of the code.

But that's already possible today without WASM. E.g. there are react plugins that render to canvas, as being used here together with some WebGL: http://www.anagram.paris/ (try to drag). And yet, we don't see this often being used. Instead, we see more DOM being used on new places (e.g. electron applications). I think if WASM+Canvas would be the way forward, we'd have already seen it by now, since it was already possible in plain JS or with asmjs if you really hate JS that much.

In the end, HTML & CSS is fast enough for most business-style applications and frameworks like angular/vue/react are pretty good ways to make frontends.

Won't using Canvas or WebGL to render all content kill accessibility?