What I really like about protobuf is the DDL. Really clear schema evolution rules. Ironclad types. Protobuf moves its complexity into things like default zero values, which are irritating but readily apparent. With json, it's superficially fine, but later on you discover that you need to be worrying about implementation-specific stuff like big ints getting mangled, or special parsing logic you need to set default values for string enums so that adding new values doesn't break backwards compatibility. Json-schema exists but really isn't built for these sorts of constraints, and if you try to use json-schema like protobuf, it can get pretty hairy.

Honestly, if protobuf just serialized to a strictly-specified subset of json, I'd be happy with that. I'm not in it for the fast ser/de, and something human-readable could be good. But when multiple services maintained by different teams are passing messages around, a robust schema language is a MASSIVE help. I haven't used Avro, but I assume it's similarly useful.