I believe the point is that the actual wire format of protobuf is not amenable to memory mapping and direct manipulation or access. So to use it this way a copy would have to be made. The appeal of cap'n'proto and flatbuffers is the ability to map and work with the serialized format with minimal overhead.

At Google scale protobuf works perfectly fine. It's our lingua franca and a lot of work (apparently by yourself included) has gone into making it performant. But it comes with normative lifestyle assumptions.

Sure you could change the wire format and implementation to be mappable; but then it wouldn't be compatible with the mainstream implementation.