What is it missing? I think only namespacing, but that isn't really necessary in JSON because it isn't a document format where arbitrary elements can appear in arbitrary places. I have never once needed namespacing in JSON.
Schemas... well there's JSON schema which is ok I guess. But in reality you probably don't want that either. Schemas are useful for IDEs and linters but that's it. Normally you validate a JSON file by parsing it, e.g. with Pydantic or Serde or Zod.
The biggest missing feature from XML that JSON should have had is comments! Yes it was a mistake to omit them.
The whole point of REST was to allow visitors to not only see the response to a request but to also be able to understand both the semantics and the functionality of it.
Think HTML - it would be comparatively useless if HTML could only contain structured data with meaning, semantics, or schemas. Anchor tags, buttons, and forms are semantic and tell the user not only what a piece of text is but how they can interact with it.
With JSON you have to know the shape of requests supported, the shape of data returned, and how to use that data to make other requests.
1: The "simpleness" made it easy for producers/consumers with unstructured languages (ie JS,Python,PHP,Ruby,etc) to participate in the ecosystem without feeling that things were "bloated" (technically a drawback perhaps but pushed popularity)
2: DTD was put into the standard, but from my recollection most stuff was defined with XSD in practice (so a separate standard), so now implementations needed to carry both deprecated stuff and another layer. (Then again, sure you could generate bindings with XSD).
3: We needed 3 versions of Swagger/OpenAPI to come to a somewhat good standardized binding API (it itself depends/extends JSON Schema), but it's there today and we don't need to consider deprecated libraries,etc and can just point our code-generators to the schema documents and get automatic bindings.
So in practice you basically have no fragility today even if there is binding generator bugs, but that feels more of an implementation issue than a systematic error, and if anything it shows that schema generators.
Now one could argue that the "free-ness" of the JSON Format made schema generators needlessly cumbersome, and maybe that's true to a certain extent but it's still not nearly as bad as trying to interpret ASN.1 files despite all the standards (there it is.. "good luck trying to find the correct IDL files").
