I'd wager there's a lot of lower level optimisations - especially for different architectures - that you just can't get with platform agnostic 'assembly'.

I also think its better for everyone if you don't need to load/download, decode, compile and run a full language runtime on every page load. Imagine a JVM running on web assembly: Every page load first has to decode and compile a full many-MB JVM, then the JVM can start booting - without any platform/architecture specific optimizations.

The more stuff that's common for webasm-targeting-languages that we can move into the browser, the less effort wasted overall.