Thanks, William. Interesting reading. However, I think you have at least one subtle misunderstanding as regards RDF. When you say, “An overarching graph model is needed, but the detailed description of the nodes can very well remain in JSON, XML, or whatever format does the job for that node type.”, I agree. RDF is an overarching graph model and nothing more. It’s various serialization formats are not RDF, the graph model is. The formats all have their limitations in serializing the model and therefore aren’t perfectly representative of RDF itself.
The confusion of RDF with the RDF/XML format caused a tremendous polarization within the community. The W3C RDF Working Group is attempting to address that in the RDF 1.1 update.