[foltik]

Because (imo) the goal should be to minimize overall complexity.

Pulling in XML and all of its additional complexity just to get a (debatably) cleaner way to express tagged unions doesn’t seem like a great tradeoff.

I also don’t buy the degenerate argument. XML is arguably worse here since you have to decide between attributes, child nodes, and text content for every piece of data.

[phlakaton]

Depends on the application, I suppose. For OP's application, pulling in XML is no trouble and gives you a much better solution for typed unions.

To get better than XML, I think you're looking at something closer to a Haskell- or LISP-embedded DSL, with obvious trade-offs when it comes to developer ecosystems and interoperability.