2. Desktop apps inherently clash with the immersive app model and UX.
While running immersive apps in desktop windows seems like it could probably work well, I'm really leery of the reverse. There are a few potential problems with running desktop apps in immersive windows that I see:
* Desktop apps can open multiple windows and draw outside of their window. Some apps (ab)use this quite a bit for dialogs, palette windows, etc. This could get pretty awkward to map to immersive windowing - do we put each in its own "strip"? Do we make each "strip" a little virtual desktop where the app can put additional windows? Do we try some mix of the two approaches, and if so, how does the OS decide which is which?
I don't see how that's a problem. Most desktop applications support being maximized and I guess some are specifically designed to be used that way (MDI applications, mostly). That's all a desktop application would have to do in Metroland: pretend it's maximized and show its dialogs and toolboxes as usual.
As for the "desktop-as-an-app" vision, it would have been easier to sell if users could have more than one desktop. That alone would have made the start screen make more sense IMO.