On the specific point of XML being for everything it was not meant to be. It was a document format and the extensible part meant you could have an island of SV

On the specific point of XML being for everything it was not meant to be. It was a document format and the extensible part meant you could have an island of SVG in your XHTML and it was all still a well formed XML. But some people ran away with the idea and extended it to uses that should have never been done in XML such as streaming.