If all you need to do is serialize data structures, we solved that problem 30 years ago—including all the schema-validation and querying stuff—with ASN.1 (and then re-solved it with Protobufs and Thrift, and solved it again in half-assed manners with JSON, YAML, etc.)
XML is a markup format, a regularization/minimization of the syntax of SGML. It's good at being a markup format—the paired named open+close tags allow for corrupted-stream repair in a way that e.g. Markdown just doesn't. XML is great in, say, DocBook.
But XML gets used for pretty much everything except actual markup. And for everything else, it's not solving those problems well.
XML is a markup format, a regularization/minimization of the syntax of SGML. It's good at being a markup format—the paired named open+close tags allow for corrupted-stream repair in a way that e.g. Markdown just doesn't. XML is great in, say, DocBook.
But XML gets used for pretty much everything except actual markup. And for everything else, it's not solving those problems well.