The complex parts of JavaScript are the semantics, not the syntax. You could reasonably easily spec a bytecode for JS to get rid of the syntax part, but nothing would change in the complexity (almost all modern engines parse to bytecode as the first step and operate on bytecode from then on).

If you wanted to implement JS in wasm, you'd either need a bunch of wasm extensions for JS semantics (dynamic object shape, prototypal inheritance, etc), or you'd need to implement them in wasm from scratch and basically ship a JS runtime written in wasm. Either that, or you need to change the language, which means de facto adding a new language since the old JS still has to stick around for old pages.