Eh, I'm not sure I completely agree with that. The taskbar in Windows 7 was already designed to reduce the distinction between launching and switching. And then even in Windows 8.1, once they added multiwindow support for store apps (a pretty useful feature I think), it sort of breaks the model in that even if there's no distinction between n=0 and n=1, there's still a distinction between n=1, 2, 3, 4 ... so there is some amount of instance management you end up having to do in some cases anyway, though hopefully not as much. (This is one situation where I do appreciate having the taskbar everywhere.)*
In general ... all these models are abstractions over 1s and 0s, right? And they're ultimately tools, they're meant to serve our purpose as users, we're not here to serve them. So I'm fully OK with "cheating" the model when it's just a lot more convenient to do so. Even back in the 8.0 Developer Preview days they broke the desktop-as-an-app model with things like Alt-Tab, because it was just really handy. However, I do think there should be a higher utility bar for features that break the model, because models are ultimately what allow people to make sense of the system (a cognitive lever). I'm not sure a lot of the 8.1.1 stuff meets that standard as is, but I think it could work better with some design adjustments.
*(Though personally I kind of think they screwed up the implementation of multiwindow a little, at least in IE - IMO additional windows in the new model should be treated as just a transient way to see two things at once for a while, rather than a persistent container for the state of ongoing activity. So there should just be one list of tabs accessible from all windows rather than each window having its own tabs - this has already tripped me up a bunch of times - and I'm not sure if offscreen windows should even each get their own slot in the left edge switcher.)