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.