Humungous amounts of business users live permanently in Outlook, they couldn't care less about the desktop if they tried.
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.