It isn’t always clear that topic maps are a value-add, not a replacement technology.
Topic maps, by virtue of subject identity and mapping rules, can enhance existing information technologies and provide reliable interoperability between them. Without changing the underlying information technologies.
Topic maps are a value-add proposition because the structures of information technologies are subjects themselves. Database schemas and their fields, for instance, are subjects in the view of a topic map. Which means that users can map, seamlessly and reliably, between a relational database and a document archive, that use completely different terminology.
Or a subscriber to several financial reporting services, can create a topic map to filter and organize those reports. That is doable without a topic map, but what happens when another report service is added? What subjects were mapped together before? Topic maps are the value-add that can provide an answer to that question.