Agreed. This is a subject in its own, and as @Helge_Hess1 pointed out, it's predominantly client-side, not server-side. The WASM compilation is during build, and not (like some isomorphic frameworks) at run-time.
Briefly @Gabor_Sebestyen: it's not transpiled. It's not JavaScript. It's WASM. As for debugging, the tools are already pretty solid for debugging the instructions.
And since when does a language wait for penetration before adopting a concept? No one's stopping front-end developers using Closure, TypeScript, CoffeeScript, etc. These are unrelated. However, isomorphic approaches to web development are becoming more prevalent due to the more singular skillset necessary and ability to combine two tiers with a common code-base. Current isomorphic approaches depend on NodeJS predominately.
The ideas behind Rust supporting WASM is to enable Rust to be a more obvious language for server-side and, inherently, web development.
If this conversation is to continue, let's move it to a new thread separately from the server group. FYI: this subject has been raised multiple times already, and I believe it's very relevant and timely to discuss it now. Just not here.