So let's give an example, where searching files in a file manager, we implement A* in Rust, then we represent the results of a search in the front-end. The DE would run in something like Electron or NW.js and windows themselves could be separate instances of the host platform.
Graphics and special compositing would work well with webgl.
What would you get for free using js. A powerful ui architecture, using just CSS and HTML to layout applications, a standard custom elements library could power the UI and react or other could work well. A host of network, authentication, Bluetooth and other APIs out of the box, potential to have your desktop in the cloud like chrome os, and many more!
Thoughts? Let me know if I am a crazzy person in the comments bellow.