Personally, I don't see a huge upsell over JSON for interop. Typing is great, but largely mitigated by things like typescript, graphql generators etc etc. edit: and json schema
The biggest issue I see with JSON today is the inability to self reference values. Data duplication overhead is high, and deserializing doesn't work as expected if you want two objects to be reference equal.
Also, nitpick: that syntax would be a grind to write manually vs JSON.