XML, XSD, XSLT, and the rest of that stack was actually useful. Overengineered and verbose, but useful - enough so, in fact, that the JSON ecosystem still hasn't fully caught up on standardizing the same feature set.
XSD mostly served to make people believe that XML standards could, as the name suggests, be extended. If you just specify your extension XSD it will work, right?
In the real world it turns out that an extended XML standard is just another standard, it may look a bit like the old one, but nothing is going to save you from writing new logic if you want to support the new standard.
While there are surely a few oddballs out there who just want to make standards for the sake it, to most of us, JSON doesn't need any "features", it makes parsing and generating data blobs quick and easy, that is all it needs to do.
XSLT (or rather XPath, and even more so XQuery FLWOR) have very expressive ways to walk the tree while filtering/mapping/folding data - much more so than JS over JSON.
XSD is just a way to define schemas for XML documents. I don't know what you mean by "extended XML standard", but islands of one schema within another were not uncommon.
