But those weren't entirely done with a webview. They were just embedding views where it made sense, like rendering a section that looks like a document (with fancy hyperlinks woooo) or render a preview of the file you selected in the main (native) view of explorer.

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.