The thing with first-mover formats (SGML and derivatives such as XML and HTML) is that they tend to stay around a long time. Where should an all-new doc format come from in this day and age anyway? The ecosystem of multiple parties adopting a format that led to XML and related standards has largely been abandoned by FAANG.

However, the threat of fubaring web media doesn't come from markup (HTML, XML, SGML) but from the complexity of CSS having gone rogue, and JS.