First, yes, the demand to be able to code natively in a browser is, I have reason to suspect, very large. It's just difficult to measure demand when nobody has ever offered a really compelling product. In engineering terms it's like getting a full-scale reading. The real level could be 1% above a full-scale reading, or the real level could be 100x higher than the full-scale reading.

Secondly, WASM isn't the right target for the browser anyway, JS and the DOM have to be what you consider "native", for the most rigorous UI projects at least (which building an IDE is). If you want to build a cross-platform UI product that doesn't require installation and has Emacs-like levels of extensibility, JS is the end of the line in terms of language selection. There are no other candidates.

Yes, in the Monaco editor (https://microsoft.github.io/monaco-editor/).

It's not just typechecking, the typescript library is also the reference parser for TypeScript and reference emit. Emitting JS from TypeScript is non-trivial and non-local.

It's not just in browsers, you might want to run the typescript library on the edge or in some restricted environment where JS/WASM is OK but native code is not.

You may want to use the typescript library as a dependency in your own package. TypeScript becoming non-pure JS means that your library also becomes non-pure JS.

I think running a typechecker on its target platforms has huge benefits.

It is incredibly useful