Agreed. JSON let's me know something is a number. That's great, but I still have to check for min/max,zero etc. A string? That's great, but I got to check it against a set of enums, and so forth. Basically, the "types" JSON gives you is about 20% of the work, and you're going to have to parse things into your own types anyway.
> What I would really like to see is a file format where the validity of the file could be established by only using the header.
Are you saying something like a checksum so not only is a schema provided but some method to verify that the data obeys the schema?
If you're talking about just some stronger shared ontology, I think that's a direction things will go. I call this concept "Type the world" or "World Wide Types". I'm starting to think something like GPT-N will be the primary author, rather than a committee of humans like Schema.org.