The forced separation of WinRT and .NET ends with the release of Windows 8.1. Now developers creating corporate apps can use any .NET library from within their WinRT apps. This is good news. It means that you have access to all your custom .NET libraries written for your existing projects. Plus you get access to the huge world of third party .NET code written during the last twelve years.
Now you can leverage your custom business layers written for .NET, and build a modern, sleek, touch friendly front end for your existing systems.