Microsoft briefly showed off a version of Office compiled for ARM running on Windows, also compiled for ARM, a very long time ago: 2008? "Oh goodie" we all thought, "MS are getting into this ARM thing heavily, we'll all have ARM desktops before too long (glad I kept the old Archimedes)". And what came out was - tada - WinRT.
What a let down.
So 13 years later they finally seem to be getting the idea.