RDF was not designed for arbitrary graphs. It was specifically designed for resource descriptions on the Semantic Web.

> reliance on the web infrastructure (IRI, DNS, web servers, ...)

This is the whole point of RDF. It is what enables Linked Data.

> I read the history of JSON-LD by one of its main author [1], and it does not originated in the RDF community.

Not sure what you mean exactly, but here’s from http://manu.sporny.org/2014/json-ld-origins/:

> JSON-LD started around late 2008 as the work on RDFa 1.0 was wrapping up. We were under pressure [...] to come up with a good way of programming against RDFa data.

JSON-LD definitely was always an RDF serialization, created by people intimately familiar with RDF and the Semantic Web.