A lot of legacy apps want to bring their party to the web, and game engines are using WebAssembly as a replacement for NaCl and Flash. Even big engines like Unreal are coming. It's a different kind of application and use case.

There's Rust, Swift, C#, Go. Plenty of people would like to(many do already) write in these languages.

I think the argument is that the set containing people who work with web UIs and the set of people who use Rust/Swift/C#/Go don't overlap very much, and generally people aren't going to learn a new stack unless there's an extraordinary external motivation to do so.

I’d amend that to “Some would like to (a few do already)…”

The user base of those languages that wants to do this is incredibly small right now