The first piece of evidence is the most obvious: When you boot into Windows 8, you boot into the Metro interface, not the Desktop. I've nosed around various options, and haven't figured out a way to boot into the Desktop, so even if it's possible, Microsoft has made it very hard to do. (If the last thing you were doing before leaving your PC was use the Desktop, and Windows 8 goes to sleep in the interim, you'll head into the Desktop when you wake it from sleep, of course, because Windows goes to the last app you used before it slept.) If Microsoft wanted to keep the Desktop around for the long term, it would have made it simple to boot into the Desktop.
Here's another piece of evidence: In Windows 8, the Desktop is just another app, run as a tile from Metro. It's clearly been relegated to the sidelines. And it's not just that the Desktop is another app, but that Microsoft hasn't bothered to spend any time improving it from previous Windows versions. Microsoft has done a very good job in designing Metro, but has essentially ignored the Desktop.
Most of Windows 8's new features essentially bypass the Desktop. As an example, consider what Microsoft calls "charms." When you're in a Metro app, the main Metro interface, a Desktop app, or the Desktop, when you move the mouse to the upper-right corner or lower-right corner of the screen, a series of "charms" appear -- icons that when clicked upon lets you perform an action, such as searching or changing options. In Metro apps, those charms are context-sensitive. So click the Settings charm, for example, and you'll be able to change settings related to that app.
That's not the case with charms that appear inside Desktop apps. Rather than let you change settings for the program you're running, you can only change the overall Desktop settings, not those for the program.
There are plenty other examples like this throughout Windows 8. As to why Microsoft wants to kill off the Desktop, there are several likely reasons. First is that it's committed to a common interface among traditional computers, tablets, and phones. The Desktop isn't suited for tablets and phones, and so Microsoft would like to see it go.
In addition, Metro has more capabilities than does the Desktop at this point, notably the way it and its apps can accept, display, and manipulate live information. Finally, in Metro, Microsoft controls what you can and can't download, and will make money every time you buy an app, something it doesn't do on the Desktop. And money talks.