Well, I don't think that holds any water. Multi-language support wasn't and still isn't a good idea; having one good toolchain and cross-browser environment is already a huge effort and far from done, supporting multiple languages would just fragment the ecosystem and slow down progress.
There is hardly any popular language today you could embed and get better performance than JS anyway, except for Lua, which barely qualifies as 'popular'.
Meanwhile there are a hundred cross-compilers/transpilers out there that give you the kind of syntax you want, without requiring a ton of changes in browsers.