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.

> 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.

calibre (with python+Qt) begs to differ. And I believe VLC runs on every current platforms. There's also Code::Blocks and Blender. Cross-platform UI is not rocket-science.