The Complexity of XML reminds me of something from Adam Bosworth's ISCOC04 Talk [0]. To me, the big takeaway is that HTML succeeded because of it's limitations, not despite of them. JSON seems very simple compared to XML. XML seems to be very powerful, but also very complex - it's like, if all you need to do is pick your kids up from Soccer Practice, you don't need the powerfullness (complexity) of the Space Shuttle in your vehicle.

  In 1996 I was at some of the initial XML meetings. 
  The participants� anger at HTML for �corrupting� 
  content with layout was intense. Some of the initial 
  backers of XML were frustrated SGML folks who wanted 
  a better cleaner world in which data was pristinely 
  separated from presentation. In short, they disliked 
  one of the great success stories of software history, 
  one that succeeded because of its limitations, not 
  despite them. I very much doubt that an HTML that had 
  initially shipped as a clean layered set of content 
  XML, Layout rules – XSLT, and Formatting- CSS) would 
  have had anything like the explosive uptake.

https://adambosworth.net/2004/11/18/iscoc04-talk/