You are completely wrong here. Please note that the EME proposal does _not_ specify a plugin interface for the Restriction module! It only specifies how the Restriction module is exposed to JavaScript. How the Restriction module is implemented or connected to the browser is up to the browser developer. Both Google and Microsoft are involved in the creation of the spec and Apple is also supporting it. All three companies make up a large share of the web browser market. And all three companies have their existing DRM solutions which they are using for their EME implementation. None of them have announced a plugin interface to allow other companies to provide a DRM module.

The only browser vendor who wants to implement EME via a plugin is Mozilla. Simply because there can't be a free software Restriction module implementation (and consequently due to the W3C's efforts there defacto can't be a fully free software web implementation). And Mozilla already made a deal with Adobe to provide the Restriction module.

So no, there won't be a market. It will actually close down the market. Content providers will have to support those four DRM solutions if they want to offer their content on all those platforms. They won't have any choice.

That browser vendors will bundle their own solutions doesn't really close down the market and I don't understand how it would necessarily hurt cross platform adoption?

Of course it closes down the market. There won't even be a market. There is not going to be a plugin interface (except for Firefox) to provide a different Restriction module.