Currently CDM decryptors are bundled with the browser, which in turn is generally locked to a particular OS or device. Browser vendors don't have to provide a way to install different decryptors and many of them aren't planning to.
Also, the actual user experience is not going to be a pure HTML5 player. Since this is intended to support hardware DRM that overlays the video onto the rendered page itself, sites have to assume the video is basically a rectangle crudely inserted into the page just like with plugins. They might be able to overlay stuff like controls on the top using HTML, but it's not clear if they'll even be able to rely on that.
Also, the actual user experience is not going to be a pure HTML5 player. Since this is intended to support hardware DRM that overlays the video onto the rendered page itself, sites have to assume the video is basically a rectangle crudely inserted into the page just like with plugins. They might be able to overlay stuff like controls on the top using HTML, but it's not clear if they'll even be able to rely on that.