It can be used to emulate a native GUI, just like languages can "compile" to javascript, and everyone can pretend it's really bytecode. But that doesn't mean this is a good idea, or that an actual bytecode for the web is just reinventing the wheel.

>Webassembly excels when it is minimized around core performance problems. Not when it is the whole app.

We don't really know that yet. Everything around Webassembly is still at the preview/POC stage, Webassembly itself hasn't matured and we don't know what sort of tooling, caching or modular integration of code would be feasible within browsers to make downloading software more efficient than just downloading one giant WASM blob.