XML underpins of a lot of standards that are ubiquitous in their areas, eg, RSS, XMPP, GPX/TCX. Tons of government APIs still use XML. In the EU, in the financial sector at least, pretty much all regulatory reporting uses XML. And when the regulators decide to make some of that data available to the public, it typically uses XML as well. Companies in many countries are required to tag their financial statements using XBRL, based on XML.

XML has not failed. It has just achieved the status, coveted amongst serious technologies, of being boring.