I love to see people advocating for better protocols and standards but seeing the title I expected the author to present something which would be better in the sense of supporting the same or more use cases with better efficiency and/or ergonomics and I don't think that protobuf does that.
Protobuf has advantages, but is missing support for a tons of use cases where JSON thrives due to the strict schema requirement.
A much stronger argument could be made for CBOR as a replacement for JSON for most use cases. CBOR has the same schema flexibility as JSON but has a more concise encoding.