The DRM plugin is for decryption only and runs in a sandbox (in Firefox anyways). Flash has a much larger attack surface. Performance of flash is usually much poorer when compared to the browser's video player.
This seems like an exceedingly fine distinction between the web as it works in theory and as it has actually worked at any point in its history since the number of browsers exceeded 1.
So easy that Twitter and Facebook are considering to implement it for all videos. So easy that YouTube is starting to implement it.
This kills the web.
Right now, YouTube is working on adding EME based DRM, as are Twitter and Facebook.
Even with their Flash based video, you could always easily download it.