In fact, they've replaced one proprietary experience with an even more proprietary one. Silverlight was available on more than one browser and OS; this is Windows-only and IE11-only. I believe Netflix are already using EME on Chromebooks but they have a different, totally incompatible DRM scheme (Google's Widevine).
But even so, the DRM system was not available cross-platform. Something like Moonlight could process the data but could not play back encrypted video using the Microsoft DRM system.
Each platform will offer a different DRM implementation, but there would be only one HTML5 player and would use the same API on all systems.
I assume that they will continue to use Silverlight on Windows prior to IE11.
