"But what would we sacrificing in openness and the web as we know it?"
Let's recap. The proposal for opened and standardized DRM method in HTML is just a bunch of callback and methods so a media content can say it has a protection and then the web browser can look up in its plugin repository if a DRM plugin can decrypt the content. The HTML part is 100% open and standardized. The actual DRM encryption and keys are not. Which is the point of any DRM scheme.
So adding DRM support into HTML, as media play/pause/method already did, won't make the Web more closed or more proprietary. The opposite is true.
Currently, media owner that choose to use protection for their content must rely on proprietary technologies. With a standard DRM framework (ie for distributing and handling protected content, not the part of decrypting it), at least, we could have much more openness on this kind of content.
Now, adding DRM to HTML does NOT change the web. Should an actor decide to use those DRMs features, you are totally free to NOT use their services. But the thing for sure is that we will have much more actors ready to use standard and open functionality to distribute their content in a protected way.