How many of these mission critical business systems are written with WPF or Silverlight for the native front-end? I don't know the numbers, but my guess is that the bulk would still be Winforms.
I think a lot of people who develop desktop front ends for .NET (and want to develop on new projects) would have put effort into learning Silverlight and/or WPF and are afraid that the effort is going to be wasted, as they need to learn another technology.