The article "Whatever happened to the Semantic Web?" is a pretty good summary of the history of the Semantic Web. A lot of the work I did for my PhD was based on Semantic Web technologies, and the article did a good job to explain how some of these technologies completely failed, while others flourished in a specific niche or use case. While the dream in the original 2001 Scientific American article didn't happen exactly as described, these kind of visions rarely do.

I feel like one area that should have found more use cases, specifically in the Internet of Things, is ontologies and inferencing. Maybe it will still happen, but I'd love to know why it hasn't happened yet.