An outside opinion... The JavaScript world is so fractured, what would you use? Kotlin, Dart, Elm, CoffeeScript, ClojureScript, Nim...? Which ones will keep going?
) The object models from other languages range from somewhat compatible to totally different. Mapping another language to good JS code is a challenge.
) Debugging and profiling become a hassle. Even with proper sourcemaps, you are just much further away from the native JS runtime.
*) Libraries. As soon as you want to use something like React, underscore, etc, you need a method to bring some stub definitions into your host language. This is solved in different ways, but always a burden. Often a very annoying one.
Webassembly right now is useful for cases where high perf is really needed (games, cpu intensive tasks, ...), but there is no access to the dom.
You ultimately need Javascript to interact with the Browser.
I recommend Typescript. It's a better language than ES(2015), but tries to remain close and compatible. It's also got enough traction and support (enterprises/big projects love it) to not be in danger of just going away.
Just want to point out that asm.js is useful for the same thing. The initial version will be roughly equivalent to asm.js anyway. Just with more consistent optimizations across browsers, faster startup and shorter downloads.