Too many man hours into optimizing Javascript. I doubt it.

I think a realistic alternative will be transpilers. Say coding Clojure, Scala, etc.. and then it convert down to javascript.

Also from what I've seen so far Mozilla really love javascript. It continues to push forward with no real alternative. Also the creator of Javascript is working with them.

I love Mozilla but I wish they tackle on an alternative javascript, they're too busy with the wonderful looking Rust though. I can't blame them.