What he fails to mention in that article, or the previous article to which he refers in the first graf, is that Metal API is only shipping in the iOS8 SDK. It's only designed for use with the A7 mobile chip. Unless Apple is suddenly going to start powering its laptops and desktops with A7 processors, Metal doesn't really sound like a comprehensive solution to me.
Is Apple hoping to back-burner OpenGL as the API for heavy-duty graphics in iOS? I'm sure that's the case. Developers have long complained that it's an utter pain to work with. What you can see from the Metal API code example is that you're working with objects in Objective-C, not a bunch of C structs masquerading as "objects." I'm sure a lot of this also has to do with Apple's overarching goal to step away from "the baggage of C," as they put it.
But the reality is that powerful, useful, and recently-introduced APIs like SceneKit and SpriteKit already rely on OpenGL to work as of right now. I don't see why OpenGL would have to go anywhere. Metal and OpenGL can coexist. But yeah, for sure, I think Metal is going to be the preferred route for graphics-intensive mobile development in iOS from now on.