https://en.wikipedia.org/wiki/EyeOS
The initial release of OS.js was around 2015, over a decade later.
Exactly my problem too, what's the main use case for a desktop in browser?
I wonder what are the fundamental features of desktop? Drag and drop, copy & paste, multiple windows being open thereby keeping track of multiple user states, better organisation of the underlying file structure, interaction centered on cursor and menus, keyboard usage focused on content input not command input, and allowing various inpedendent applications from interacting together via a unified view, unified file structure and unified visual shortcuts.
I think that a OS in the Browser would be great if it would allow websites to visually interact with one another, just as desktop OS's allow various applications to interact visually - via drag&drop for example. Then it would make sense for me to use an OS in the Browser.