I'm not going to disagree with your comments on WinForms/WPF or current limitations for WinRT. Let me focus on WinForms/WPF for a minute...
First, I'm curious if your client application needs to support Windows 7? If so, the choice is clear for a Windows platform, WPF. If is the technology that is alive and staffed by the same team building future investments into the Windows client platform.
I was a bit surprised to read this. I do recall this: http://www.zdnet.com/blog/microsoft/microsoft-splits-up-its-xaml-team-whats-the-fallout/9807 which suggests there are multiple teams doing something with xaml, but I learn from your email that this isn't the case?