Absolutely with you up to here, but...
> Particularly these days when you can have a coding agent write the parser boilerplate, etc. for you.
Absolutely not. Having seen the infinite different ways a naive implementation of XML goes wrong, arguably being one of the main causes of death for XHTML because browsers rightfully rejected bad XML, "Don't roll your own XML implementation" should be right up there with "Don't roll your own crypto".
I don't feel like it's going out on a limb to say that if someone needs to defer to a LLM to implement XML they're not qualified to determine if it's done it right and/or catch what it got enthusiastically wrong.
The only example I can think that messages are expressed as documents is Microsoft Teams. And it’s as much an example of what not to do as anything.
A self-contained rich formatted text file/package that optionally contains attachments or media.Very few messaging apps let you go beyond plain text and let you start embedding media or complex layouts inside a message.
Pretty much no messaging apps let you create messages more complex than markdown anyway.