Haven't looked at the APIs, but are the APIs going to scale between phone, tablet and 30" 4K monitors? It's relatively easy to make the same app to be adaptable between phone and tablet, or even laptop, but to make it look good on 30" 4K monitor is a completely different UX, if full screen. Or in high resolutions the apps run in windowed mode?
You can adapt any part of the app. I.e. you can simply migrate the app to be universal across devices, but if a specific part does not look good, then you can change the XAML/code behind for that part.
But do these "universal" apps still run full screen on 30" monitor? If so, then it's not a good UX. The benefit of a large high resolution monitor is to display lots of information at the same time, this frequently implies more than one app/window visible at the same time.
Yes, unless I am missing something. It's up to the developer (same as on Web w/ CSS Breakpoints) to accommodate the various 'breakpoints' or devices visually. To say it's poor UX is inaccurate, will there be plenty of unoptimized UI/UX full-screen apps? Sure, but the functionality itself is not poor UX.
XAML and its Adaptive Design concept allows the layout to rearrange itself to better fit smaller or larger screens. And the programmer can always explicitly use different UIs depending on how many pixels are available to the app.
And everything I wrote is already happening (actually, it's been happening for a while). Just try any well-built W8 app in different screen sizes or snapping configurations. ;)
