It did okay for its time .. it has some big advantages over JSON today; namely comments and namespacing. The namespacing made it easier to use to marshal and serialize data with type checking. SAX parsers can handle streaming XML and they're very fast. Certain protocols in the open source work still use XML (like XMPP).

...I dunno ... JSON feels like a mistake ... but not in an of itself. Javascript was the original sin. JavaScript is the devil; the fall of all mankind.