Surely this is a product of the fact that XML is just more extensible (it’s in the name after all).

If you tried to represent the data (exactly) from any of the examples in the post, I think you’d find that you’d experience many of the same problems.

Personally, I think the problem with XML has always been the tooling. Slow parsers, incomplete validators

Maybe rather: how easy it was to generate rotten XML. I feel you there.

The XML community, though, embraced the problem of different outputs between different producers, and assumed you'd want to enable interoperability in a Web-sized community where strict patterns to XML were infeasible. Hence all the work on namespaces, validation, transformation, search, and the Semantic Web, so that you could still get stuff done even when communities couldn't agree on their output.