Am I missing something, or hasn't Microsoft done this since Windows 9x with apps like Explorer and Control Panel heavily using web views internally rather than "native" WinAPI GUIs?
Now we are talking about entire apps being built with that stuff, down to the window border (or lack of it). It's impossible to have a consistent looking and working OS with this approach. It's impossible to share code between these things and the actual native apps, and often things have to be written from scratch and end up using 10x memory than the native solution.
Not very useful because you quickly realize you mostly obscure your desktop with actual applications you want to use on your computer.