EME doesn't prevent browsers from implementing EME. But since EME doesn't describe how the browser should talk to the CDM (just how scripts on a page talk to the browser), implementing EME is not useful in terms of working with actual CDMs that exist in the real world...
it locks out new browsers because acquiring CDMs can be a pain if you want to roll-your-own
https://hacks.mozilla.org/2014/05/reconciling-mozillas-missi...
ctrl+f for "What does this mean for downstream users of the Firefox code base?"
tl;dr you either need to run a mozilla-built sandbox with no modifications, or literally call up adobe and ask about being given a CDM for your own browser, which they might just scoff at
As I understand, EME doesn't lockout new browsers, it locks out new architectures and kernels. But any new architecture that comes along that anyone would want to use to view DRM-encumbered media would be mainstream enough that content providors would support it, and people running an experimental OS kernel already have to use a more traditional system for a lot of things, and probably aren't a demographic too keen on DRM in the first place.