I hate the X and L parts too. Just because you put a URL inside doesn't make the other side understand your structure. The features that try to make it extensible actually make it less so.

I can't think of any cases XML has helped, and plenty where it's massively gotten in the way. XMPP should've been json for instance. React used something like XML in structure for JSX but didn't actually use XML, so thank goodness we didn't have to put xmlns= all over it.