But I think it's much more worthwile to dive into the particulars of why it failed. While they're all fads, some of them share commonalities that, in hindsight, I think we can say were major catylsts to their downfall.
With XML, it failed because something like JSON was much simpler. Time and time again I see people saying that json+comments would be the ideal format. Time and time again we see that people prefer simple solutions that they can get jump into easily. Maybe certain problems don't require solutions with those traits, but anything with sharing data among a diverse group of consumers does.
The XML ecosystem with things like XSLT, Xquery,etc, were elegant, but also overengineered and clunky. I would even say that markup itself with closing tags were more annoying than just closing brackets.
Software is still so young... I think it's good that we tried and learned something from it in the short term. Maybe one day we'll get a proper standard with all the right data types that becomes the defacto solution that people learn it even if it's more complicated.