> developers must become domain experts [my emphasis] in a rich and complex space that is essentially unrelated to the application itself.

XML is a markup language, but most people that used it just needed a standard structured data format. In comes JSON which is more easily compatible with the object systems of various languages and in particular is compatible with Javascript syntax, and XML loses most of the people that used it.

As a markup language though, it seems pretty good. It's just that the amount of people that actually need an extensible markup language is much smaller.

I do hate the strictness of it. The header

  <?xml version="1.0" encoding="UTF-8"?>