The author makes an argument against designing new XML languages. I think his arguments are weak. This does not mean I think we should design more XML languages, but that the arguments this particular author brings against it are weak. That having be said, the mid section with the tooling suggestions by use case is neat.
One thing he condemns such endeavors for is that it is unpleasant and somehow "political". I can see what he means, but this has nothing to do with "overdoing the extensibility" of XML. As Aaron Schwartz put it
"Instead of the "let's just build something that works" attitude that made the Web (and the Internet) such a roaring success, they brought the formalizing mindset of mathematicians and the institutional structures of academics and defense contractors. They formed committees to form working groups to write drafts of ontologies that carefully listed (in 100-page Word documents) all possible things in the universe and the various properties they could have, and they spent ours in Talmudic debates over whether a washing machine was a kitchen appliance or a household cleaning device. [https://www.cs.rpi.edu/~hendler/ProgrammableWebSwartz2009.ht...]"
It is true that similar endeavors are prone to looking for an Absolute Cosmic Eternal Perfect Ontological Structure (credit: Lion Kimbro). If you drop that idea in any office, you will get as many proposals for entities as there are anuses, as if anyone is entitled to an ontology.
Don't get me wrong, anyone might be entitled to submit an entity or criticize a hierarchy, but I think this is meaningful mostly in the context of targeted audience research and agile development practices. All in all, I think that the problem here is not with the 'X' in XML, but with poor organization-level practices.
Furthermore, I did follow the link and surveyed the XML languages. I did not see the apparently self-evident truth the writer sees in there. Sure, there are many of them, but how is this even an argument? Some of the listed languages seem quite cool to me, especially the science ones. And the next person might dig the legal ones. If the argument here is that "there are so many of them languages, they just can all be important" (or "real") does not sit well with me. There are tons of different programming languages, web frameworks, linux distributions, not to mention the incomprehensible multitude in other domains, such as car maker models or, well, birds.
It is just simplistic to disparage any number of things because they are too many to make readily sense of, and this is a cognitive stance I can't endorse. Look at Medical Subject Headings, or the Dewey Decimal or the Library of Congress cataloging systems. There is just a ton of things out there and for each one of those, there is a person that has more expertise on than yourself. These taxonomies might be important to them, what are you gonna do? Stop them?
A bird's view exasperation of the sheer number of things is the hallmark of a small town mentality that is untenable for the hacktivist mindset. The response here is, I guess, reusability of existing standards, and agile practices involving the user in the development process. But the author did not bring up any of these.