XML in XMLHttpRequest was first superseded by HTML and then by JSON. And JSON only because the JavaScript API for XML (aka. the DOM API) is the worst thing imaginable compared to the simplicity offered by JSON.parse and then the JS native representation of it as objects. The same ugly DOM API let to the advent and rule of jQuery.
The success of JSON has less to do with verbose/clunkiness but a lot with the available APIs in the JavaScript frontend.