Which for me is a design smell, why bother with workarounds when XML already offers me all the best tooling out of the box.

For me, xml is just too much, and when working with C or C++ (not t sure about rust) it's just a pain. Can your json schema file not serve as the documentation?

Why should it be a pain? Parsing XML is no different than parsing JSON, there are plenty of robust parsers available since around 2000.

Mature libraries with plenty of experience deploying into production.

Plus for small enough stuff, like configuration files, one can just plug XPATH as well, making the search for items even easier.

If something like k8s implemented comments as specific comment fields that would actually be pretty useful. The fields could be parsed and show in GUIs.

That is the workaround, just add it as "comment":"comment text". Having one comment for each field in an object would get unwieldy though. Anther place for them is possibly in the json schema file.