And yet, just yesterday there was a great discussion on Virtual Desktop Infrastructures, where entire operating systems are accessed and operated virtually through just the browser [0].
The current top comment indicates that while there are some setup hoops to jump through to use a specific OS, the performance itself "works very well" [1]. Does this not qualify as a web app replacing a client entirely?
[0] https://news.ycombinator.com/item?id=11721466 [1] https://news.ycombinator.com/item?id=11722141