It seems to me the future is rather in gradually replacing JS engines with a WASM back-end. The whole webpack/babel ecosystem could be replaced by a toolchain which targets WASM, and browsers could ship with a JS engine which just translates es5 to WASM for legacy support.

I guess the WASM api is lacking a lot of features which would be required to make this work, but I don't see why it shouldn't be the goal.