It's hard to speculate how the culture and philosophy of web development will change, so let's leave that aside and consider what's possible to do. It's certainly possible to compile into wasm a desktop program in a high-level language, even a program with a GUI. When the browser runs that wasm, the user will interact with your GUI instead of HTML elements. See webassembly.github.io/demo.

More excitingly, I presume it will one day be possible to compile a mobile app into wasm and execute it in a browser, achieving instant parity between mobile web and apps.