It's been 84 years but I still miss some of the "basics" of XML in JSON - a proper standards organization, for one. But things like schemas were (or, felt like) so much better defined in XML land, and it took nearly a decade for JSON land to catch up.

Last thing I really did with XML was a technology called EXI, a transfer method that converted an XML document into a compressed binary data stream. Because translating a data structure to ASCII, compressing it, sending it over HTTP etc and doing the same thing in reverse is a bit silly. At this point protobuf and co are more popular, but imagine if XML stayed around. It's all compatible standards working with each other (in my idealized mind), whereas there's a hard barrier between e.g. protobuf/grpc and JSON APIs. Possibly for the better?