Locking out a massive user base (those who have used desktop forever and still need to) will not be easy. ... Even Apple would have failed if they want/wanted that fully on Mac computers.
Uh, Apple does it all the time. Remember the transition from 68000 to Powerpc?
From classic os to os x?
From Powerpc to x86?
Just dump the old API. Provide a crude emulator. Let the PR machine roll and tell the story that the emulator works marvellously, and you shouldn't need it anyway.
Wait and see that the PR machine of MS telling the story of how you don't need legacy desktop applications anyway, and if you need them, there is a virtualization solution in win9.
Granted that Microsoft does not have the hypnotism powers Apple has over the media, this may fail, but the failure is far from certain.