James Clark is a big name in the XML and SGML world. He was the technical lead in the development of the XML 1.0 standard, co-author of the XSL standard, editor for the XSLT standard, and co-editor for the XPath standard. He was involved in the development of the extended version of SGML on which XML was based, and he wrote sgmls and SP, the two most widely used SGML parsers in existence. He was also the person who coined the name "XML" in the first place.
According to him, XML is dying out on the Web, and for good reasons.
Ultimately, he boils this down to a growing need for simplicity: XML is slow to parse, complex to use, and keeps getting harder and harder to comprehend. For a number of years, there was a crude running joke among web developers: "XML is like violence: if it doesn't solve your problem, you're not using enough of it." To many web developers watching XML and related technologies evolve, that was the basic impression of the W3C's philosophy around XML. By the middle of the decade, the increasing complexity of the standards led developers to become disenchanted with the whole stack.
This sentiment is shared by others in the industry. The CEO of Digital Bazaar, a web application development company, wrote an interesting blog post which touches on many of the same points. His company had formerly used SOAP (XML) for all of their web services before eventually switching everything to REST/JSON to cut out unnecessary complexity.
What prompted both of these posts was the news that Twitter and Foursquare both had dropped support for their XML APIs in favor of JSON. This follows similar transitions by Google and other prominent web companies, in an effort to make their services easier to implement and consume.<>