The URL is just an opaque path appended to a domain. You can just as easily use an UUID instead of an hierarchic model. No component in the system gives a damn.
In other words, you want to be able to walk into any library and use ISBN as the underlying identifiers, instead of the Dewey system, which is not broadly applicable. You might get the Dewey number from the ISBN and vice versa, but the ISBN resource assigned to a book resource is the fundamental linking pin.
So, when we link from an article, we want to be able to say: "it is this specific article I am linking to, you figure out how to get access to it." This will be much more durable than an ephemeral URL.