I've done similar things in the past (json over udp-to-localhost, with a schema description used to generate the code for both parsing and generating on each end). It's a totally reasonable point in the design space, and I literally never saw a reason to revisit the decision to use json for that system.

You'd do that because everything under the sun knows how to generate and parse json. Performance looks like a total non-issue for this use case. The messages are tiny, local and infrequent. You wouldn't want to do this on some kind of a high performance application on the data plane, but on the control plane it's totally fine.

Even if you expect that all production use cases will use some kind of higher level library that does schema validation, it can still be quite nice during debugging to a) be able to inspect the messages on the wire when debugging, b) to be able to hand-write and inject messages.