What happens if you need to dynamically discover a service and invoke it? Or does it only work if both ends are known and you can push out change to both end of the wire?
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.
I always try to avoid these special formats. Most people have not used them and even if there are claims that the tools work on different platforms there are often some quirks. Another approach is to add compression and tweak it for performance.
Also, wouldn't something like MessagePack, ION, RION, or CBOR give you payload compression over JSON?