The EME proposal by the W3C is specifically intended to enable just what you describe: proprietary plug-ins that only work in particular browsers.

One of the primary sponsors of the EME proposal is Netflix, who are already building exactly what you describe.

I've addressed this issue - a misconception that at least some on the W3C hold - in detail here:

http://www.w3.org/QA/2013/05/perspectives_on_encrypted_medi....