I never saw XML schema as a bad idea, just overdesigned and verbose, same as all things XML.

I happen to think otherwise regarding XML, and rather enjoy using all high level tooling for XML processing.

As opposed to underspecified, and ill thought out?

To me it seems the XML era had better engineers making the standards.

relax ng?

Well, JSON Scheme has "title", "description", and "examples" keywords that are meant to be used for additional info, so not having comments is not really that big deal as with plain JSON. For XML Schema comments are essential as it's super verbose and hard to read. Of course, if you use it a lot you'll with time train your brain to scan through the forrest of XML tags without effort, just like in 90s we were able to easily make sense of multi-level nested tables - but to untrained eyes it's still a mess of tags with million options and it's the main reason why XML lost the popularity - too much noise and verbosity.

JSON Schema has "$comment" as of draft-06 or -07 (I forget, which is funny b/c I was the one who added it).