That's a good point - markdown did take the 'readable-by-humans' use case that XML never quite nailed. Though I wonder if the comparison is level-appropriate: markdown is a syntax for simple formatting, while XML was attempting to be a universal interchange format with schemas, namespaces, validation. Markdown won in one niche by being 90% good enough; XML lost by trying to be 100% everywhere.

Unfortunately Word documents are XML. Microsoft has a LOT of customisation going on, but at the core it's very ugly, incredibly complex xml :(

Source: https://learn.microsoft.com/en-us/office/open-xml/word/worki...

XML is still the implementation tool for Microsoft Office and Open Office docs. I wouldn't hold those up as the gold standard or anything, but it's hard to see how Markdown could capture everything that XML does for, say, powerpoint or excel.

There's also HTML, LaTeX and Typst for documents. I don't think that there is a clear winner here.