XML is much better than JSON for document-oriented data like messaging and web pages. Use the right tool for the job.

IM messages aren’t really documents. They are text with some very minimal formatting that could be expressed with markdown. Any media attached isn’t embedded in the document, it’s attached externally / rendered at the bottom.

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.