WebAssembly AFAICT is all about making existing code runnable in a JavaScript environment.

I would not say that. It is one aspect of web asm, but not the only or even main reason. There are far more frameworks and tooling out there designed for writing new stuff in it rather than getting existing stuff to run.

if you want to write WASM in a strongly typed language, you need support libraries that define all of the browser primitives.

These already exist for some languages - notably rust (but not exclusively). You can now write a web app fully in rust and have it preform on par with popular JS frameworks. There is JS involved in this - but it can all be generated so developers never need to actually touch it.

If you’re an accomplished web developer, it’s more effective to stay in js.

This is true, and the bigger reason why wasm is not as popular. If you already know a JS web framework there is little point in learning a whole new language as JS is good enough for most applications. And a whole new language and tooling is a large investment. But it does mean that people in other eco systems or that don't really like JS are now able to write webapps in other languages (which by the very nature are going to be a fraction of web devs as these people will likely have avoided it where they can). Other eco systems are also a lot less mature than JSs eco system for web development.