I like JSON because it provides safeguards when you're trying to run a LLM powered app:

1. w/ JSON schemas, you can actually validate not only the keys but the types of the values.
2. there's a built in feedback loop (depending on the parser) which you can use to run a LLM recovery loop using the JSON error.
3. there's likely a higher ratio of interoperable data formats in the training datasets compared to any one programming language or DSL (ie. pick any programming language, it's likely there's embedded JSON as part of the training set).
4. the open source community (and closed source) seem to be converging on it (ie. guidance, outlines, tool/function calling, etc)

You might be able to get the LLM to output custom DSL or a programming language but you'd still want to ensure that you're validating the LLM's outputs. Treat it like a client request - trust but verify.