The auto-generated SDKs are very useful here. An API customer doesn't have to learn protobuf or install any tooling. Plus they can fall back to JSON without any fuss. Connectrpc is much better at that than my envoy transcoder was.

If you're thinking from the API author's point of view, I might agree with you if there was a ubiquitous JSON annotation standard for marking optional/nullable values, but I am sick of working with APIs that document endpoints with a single JSON example and I don't want to inflict that on anyone else.

It doesn't use protocol buffers any more than gRPC does, which is to say it only uses them if you choose to use them. gRPC is payload agnostic by design. Use CSV if you'd rather. It's up to you.

You can also choose to use JSON instead. Works great with curl and browser dev tools.