On the contrary I think they did the right thing in not supporting any schema it.
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").