And Outlook and Office aren't desktop programs? What about the tons of inhouse applications? I guess there are more VB6/MFC/Win32 applications in corps than .NET ones. Do you think they will convert them over to Metro in a few years?
Humans will colonize Mars sooner than Metro apps being in the majority in corps.
You click Outlook on the Start screen, Outlook launches, you do your work. Interaction with the desktop environment is often actually quite minimal. Where it does occur is in situations like attaching files and so forth, which is often an area that causes the most problems currently.
I don't expect most of the VB6/MFC/Win32 corporate apps to be moved to Metro in the next few years, most likely they will be moved to web applications as that has certainly been the trend in corporate application provision for a long time (hence the heavy ties to IE6 we've seen). Web applications will naturally run in the Metro IE world quite happily.