At ESTS Antwerp recently (5-7 October 2016) some XML aficionados thought that the StackOverflow graphic in my previous post was somehow misleading, that attendees at the conference needn't worry about the decline of XML, because it wasn't really happening. But they didn't offer any facts to counter the evidence.
XML Web services
Five years ago in 2011 on ProgrammableWeb there was a news story posted based on APIs submitted to their index, that stated: 1 in 5 APIs say "bye" to XML, or, 1 in 5 APIs were already JSON only: that is, they offered no support for XML.
What's an 'API'? It's an index into the functionality offered by a web-service. Typically data is sent to the service in some format and returned via some other (or the same) format, such as XML or JSON.
The 2011 story was widely quoted, so I went back to the site and created my own analysis of all JSON and/or XML APIs in their registry from 2005 to October 2016. That's a total of 4,453 APIs. Since 2011 1 in 5 has now become 4 in 5:
According to the effective creator of XML, James Clark, Web services were the biggest motivation for XML in the first place. The disappearance of such a big usage case for XML will inevitably result in the withdrawal of vendor support for XML products and open source development projects that they patronize. Without the support of key open-source XML building blocks, which are not being adequately maintained as shown in the graph below, commercial new products based on XML will no longer be possible, and existing ones will break.
Releases of 8 key open-source XML tools1
General popularity of XML
Another possible source of information about xml's decline can be found in the archives of xml.org, which is sponsored by industry giants IBM and Microsoft, and hosted by Oasis. The xml-dev newsgroup documents a marked decline of developer interest in XML since its inception, as this graph of the number of monthly posts to the group between February 1997 to October 2016 clearly shows:
A corroboration of this trend can be found in posts to the popular Slashdot.org news site which mention either an XML language or XML itself:
XML 'Mixed content'2
The use of XML for mixed content seems likely to succumb to the same trend eventually. Its decline is evidenced by falling interest in DocBook and TEI. Unlike SGML, XML never was designed to be typed manually, even in an XML editor. While interest in DocBook has plummeted to 1% of what it was 10 years ago, simpler markup languages like Markdown have risen dramatically in popularity. Niche XML vocabularies like TEI would thus seem to have no future; their survival will depend on the continued maintenance of XML tools by a tech community that is rapidly losing interest in them.
Popularity of DocBook (blue) vs Markdown (red) 2004-present
Interest in XML document databases like Sedna, BaseX, eXist and MarkLogic (now a dual XML/JSON database) is roughly flat, but at a very low level. For every user of exist-db (XML document database) there are 3,500 mongo-db (JSON document database) users. The only possible reason to choose an XML over a JSON document database can be that the data is already in an XML format. All four surviving XML databases show a slight decline since 2014, probably due to companies gradually migrating their documents to other formats.
Native XML databases popularity 2012-2016
XSLT transformation language
XSLT, the XML transformation language, seems to be dying even faster than XML itself. The clumsiness of its syntax, combined with its inability to directly transform HTML5 (although it can write it) has doubtless contributed to its demise.
 Cocoon, fop, libxml, libxml2, xalan-c, xalan-j, xerces-c, xerces-j
 'Mixed content' is when tags may appear in text and text in tags