The UI is different on a touch-tablet. Not on a desktop, or server, or your regular PC.
For everyone else, you get into Desktop mode by clicking on the desktop tile, that's right there in your face after bootup. One click.
The desktop is now an application in itself, but I'm not sure if I'd call the desktop a legacy app for legacy apps. There are plenty of workflows, stories, and apps that simply can't be reduced to a Metro type use-case.
Android tablet market is also minuscule, despite Android UI fitting tablet use cases.