Earlier quoted context omitted.
But no matter what they can decide that already... User agent checking (and other fingerprint checking systems), custom plugins which are required, or even going around the browser entirely and requiring a downloadable program or app to playback. If the standard doesn't have EME, then alternatives will be made which will be much worse. EME is the best possible outcome in a world where DRM exists, and I would be just…
Well, historically all those mechanisms have been problems for the open web. To me, the question is why you couldn't achieve the same thing without making the CDM a blackbox, e.g. just by issuing license keys and standardizing the encryption/decryption of content. It seems like the only benefit to making CDMs blackboxes is that it gives publishers more control over the user's environment.
That locks me in to your encryption system, and probably locks me into other things as a result. That also doesn't give me the ability to enforce other DRM things like only one playback per device, or only allowing it to happen between 7am and 7pm, it also means that i need to trust the client, whih is exactly what DRM is trying to not do (because if the publishers could trust the client, an HTML flag for "don't let them save this" would suffice.)
DRM by nature basically needs to be a black box. This just standardized how that black box hooks into the system, and sandboxes it to limit what it can look at (and so we don't have a repeat of the Sony music issues)