Microsoft needs to dogfood it. By porting MS Word, Excel, Powerpoint, Outlook and Visual Studio to the WinRuntime and XAML Framework would a big step.

For example no high profile MS client product is written in C# using the dotNet Framework. All parts of Windows (kernel/user mode, shell), Office and various applications are coded in native C or C++ against the WinAPI. Visual Studio is mainly coded in C++ too, with some dotNet bindings. Some server products (outsourced to Microsoft India) use the dotNet Framework like SharePoint and Exchange.

It would show confidence in their WinRT API. WinAPI will be around for a long time and even WinRT-API is based on WinAPI/Win32 subsystem and COM.