AIUI a “markup” language is for applying structured enhancement to a document containing effectively arbitrary contents. An unstructured source, but we need to add some structured components. Obviously today it has all reverted to a structured document object model, but one of the insights of HTML was, at least in the era of gopher and ftp, the system shouldn’t interfere with the data more than it needs to. If I write up a nicely formatted RFC text file, and I want to put it on the World Wide Web, I shouldn’t have to translate the whole thing to some other language. With HTML, I could, in a structured way that remains independent of the structure of my document, add enough bits to make it work with the World Wide Web. I could, in other words, take my already formatted and structured document, and mark it up with metadata.

At the time this “markup” concept got everyone all excited. But it ended up not being the way to solve problems like what you seem to describe, which don’t have the arbitrary document aspect at all.

Which is exactly my point. Markup and structured data are different. Even at the time of XML’s reign, there were plenty of established ways to serialize and exchange structured data.

Trying to solve a structured data problem with a markup language is how the markup language as a concept got contorted into a data structure system that was simultaneously both too flexible and arbitrary and too maddeningly rigid.

JSON solves it by being a data structure first and last.