Note that all four systems compared have the ability to compile schemas into self-hosted structures. E.g. Cap'n Proto has schema.capnp which defines a Cap'n Pro

Note that all four systems compared have the ability to compile schemas into self-hosted structures. E.g. Cap'n Proto has schema.capnp which defines a Cap'n Proto structure representing a Cap'n Proto schema. Code generator plugins actually take this structure as input, and the parser is available as a library as well. So you can actually write tools that programmatically read and manipulate these schemas pretty easily. It's certainly a matter of opinion, but I find XML- and JSON-based schema languages extremely difficult to read with all the boilerplate they end up having.