How is shrinking the surface of the required proprietary, platform-specific binary blob from "the full application stack of Flash or Siverlight" to "just the DRM bits of Flash or Silverlight" not a step forward?
The CDM API should make porting to multiple platforms easier because the browser implements all the non-DRM infrastructure like networking and playback. That said, I don't really expect Linux to suddenly be a top priority for Netflix or Amazon. On the other hand, a CDM is bright red bullseye for crackers. :)