(1) some operators only care about a handful of the URLs under their domain;
(2) hardly anyone uses link relations, so most links are devoid of semantic metadata and are essentially context-free, requiring a human to read the page and try to guess the purpose of the link;
(3) so many 'resources' are now entire applications, and the operators of these applications sometimes find it undesirable to encode application state into the URI, so for these you can only get to the entry point -- everything else is ephemeral state inside the browser's script context.
But I disagree with the statement that "the reason for the eventual demise of the URL will simply be the fact that the concept of 'resource' will just not be sufficient enough to describe every future class of application or abstract behavior that the web will enable."
URIs are a sufficient abstraction to accomodate any future use-case. It's a string where the part before the first colon tells you how to interpret the rest of it. It'd be hard to get more generic, yet more expressive.
The demise of URLs, if it ever comes to pass, will be due to politics or fashion: e.g. browser vendors not implementing support for certain schemes, lack of interoperability around length limits, concerns about readability and gleanability, and vertical integration around content discovery.