Your complaint is with the semantics of the particular file. There's no reason why files in /dev need be deletable using unlink. That's an historical artifact, and one that's being rectified.
"Everything is a file" is about reducing most operations to 4 very abstract operations--open, read, write, and close. The latter three take handles, and it's only the former that takes a path. But you're conflating the details of the underlying filesystem implementation with the relevant abstraction--being a file implies that it's part of an easily addressable, hierarchical namespace. Being a file doesn't imply it needs to be deletable. unlink/remove is not part of the core abstraction. But they are hints that the abstraction is a little more leaky than people let on. Instantiating and destroying the addressable character of a file poses difficult questions regarding what the proper semantics should be, though historically they're deletable simply because using major/minor device nodes sitting atop the regular persistent storage filesystem was the simplest and most obvious implementation at the time.