That design decision makes sense, to decouple the EME module, and great that Mozilla offers a prebuilt binary without it, as an opt-out. I wonder if Chromium (or a fork) can also be built without this feature?
I'm not sure what decoupling means, and whether it's of any technical significance beyond bragging rights, but chromium won't support drm out of the box either because widevine is obviously not a part of chromium. You can take the widevine library from chrome and make it work with chromium if you jump through some hoops.
Not sure how it works on other OSs, but in my experience on Linux, Chromium is installed without Widevine DRM, Flash, or any proprietary stuff, and if you want that you have to install it separately.