Also, wouldn't something like MessagePack, ION, RION, or CBOR give you payload compression over JSON?
One thing about protobufs in a highly interconnected ball of mess, good luck reving the protocol in any non trivial way. Schema-less encodings such as those you mention (as well as bson, etc) are really advantageous for loosely coupled interfaces and graceful message format migrations. IMO (opinion!) protobufs is popular simply because of the Google cargo cult where anything Google tech is slavishly adopted without a great deal of introspection on applicability to the specific situation.