The tragic part isn't that the schema is given in XSD but that it's defined in XSD. What I'm lamenting (yes more hyperbole) is that a schema this important wasn't defined in a schema language that had clear semantics and composability rules, fostering reuse and adoption for related domains, of which we would expect there to be many. (Later, of course, the definition could always be extruded into XSD and other popular yet semantically stunted formats as a practical publication step.)