Don't you think that someone is eventually going to compile a JVM to wasm which would allow languages that compile to JVM bytecode to run directly as standard JVM bytecode in the browser? Wouldn't that allow to have as good performance as JS compilation? (I am asking you as it looks like you might have some expertise on languages that compile to both JVM and JS ;))
It might allow you to have as good performance as JS compilation, but definitely not as good interoperability with JS. Some of those languages, like ClojureScript, Scala.js and BuckleScript, have complete 2-way interop between them and JS, including for mutable objects, their properties and their methods.
"Just" compiling Scala to JVM-on-wasm does not give you the real power of Scala.js, which is its interoperability with JavaScript libraries. Similarly, just compiling Clojure to JVM-on-wasm does not give you the real power of ClojureScript.
People often forget about the interop with JS part--which is immensely more important than raw performance--if they don't actually work with a language that offers it. :-(
