We are talking about the new Windows 8 programming model. Any app/application that follows any other model is considered legacy if you go by what MS told us last week. That is a huge implication.
I think we are almost on the same level, but we both look differently at the message Microsoft presented with that youtube video. To me, they didn't tell at all that WPF or Silverlight would become legacy. You could still run those applications and I assume that during the development of Windows 8 this experience within windows 8 will improve. I think xaml based development will be intergated just as well as html5/js, but just isn't today.
I believe the form factor and usage of a pc is moving away for the desktop pc. More and more end users get a laptop, slate, tablet or any more mobile device. These devices are always connected and less powerfull than a fresh desktop pc. What Microsoft does with this step is allowing those users to use their apps. So that there marketplace of availible apps gets bigger (more html devs than silverlight devs and easier porting of apps). These apps are the apps they use to (if I look around me): facebook, twitter, whatsapp, email, watch video, manage agenda, photo browsing, weather info, browse web, etc. When looking at one of those future videos, where everything is a display, i mainly see usage of communication and information gathering, which are the apps that come back in the list of apps above.
I think, while it is a different form factor, that with those more mobile 'always connected/ always on' devices, the touch friendly interface, and a large marketplace of apps this becomes real and is what they presented in the Windows 8 video. And most of those apps can be build using html5/js. I don't know where a device like the xbox fits in here, because the tv feels to big to me, I see it more as a device to watch media on and play games on or call with family, like you do with skype. Maby for users of the current teletext http://en.wikipedia.org/wiki/Teletext, this feels different.
Next to that usage, some might still have a desktop pc, to run what you might sooner call 'real' applications. Like the one you linked to, advanced photo editing, visual studio, video editing, 3dsmax, that sort of applications. Those don't work that well on a lower powered device and in some case don't work for touch. But they are in no way going away, and building them as html apps is indeed insane.