This seems reasonable, but I'd like to add that XML, while certainly not designed as a config language, could be made much more terse by using SGML, the markup

This seems reasonable, but I'd like to add that XML, while certainly not designed as a config language, could be made much more terse by using SGML, the markup metalanguage XML is derived from and of which XML is a subset. SGML has additional constructs for short forms such as tag omission/inference and short references (custom Wiki syntax) targetted at authoring, whereas XML only admits canonical angle bracket markup syntax which is what makes XML verbose and cumbersome for edititing by hand.