It should have been, but just like the masses rejected LISP for its parens, the masses rejected XML for the closing tag. We could have avoided PHP and the zoo of MVC frameworks.
External entities, namespaces within namespaces, CDATA, namespaces that confusingly look exactly like URLs, but aren't, parser vulnerabilities.
The bad smell was mostly coming from all the things inherited from SGML - CDATA, entities etc. Although I would also add that separation into elements and attributes is not a particularly convenient way to model many things; it mixes up differences that should really be orthogonal (ordered/unordered, atomic/composite etc).
In my schema, I have a <foreignDocument> element, whose contents is defined to be a foreign document.
Yes XML is more expensive to parse, but so is all the extra stuff you have to do to get around the various limitations of JSON, which could have been done cheaply by an optimized XML parser.
I doubt that. Javascript frameworks have their value inside a corporation - they enable replaceable cog worker programming. The result sucks, but hiring and replacing React programmers is easier.