I think it's less that protobuf is a moving target, and more that gogo tried to add in all the features that google didn't want to maintain, and learned that maintaining a massive feature matrix was impossible.
GoGo was not a completely separate implementation but deeply hooked into the official GoProto implementation. So it wasn't "Protobuf the binary wire format" or "Protobuf the schema language" which changed over time here, changes to the Google's Go library caused it problems. It's like building a library that integrates with Jackson (a JSON library) and Jackson details changed in ways that added toil, versus JSON changing.
I think it's less that protobuf is a moving target, and more that gogo tried to add in all the features that google didn't want to maintain, and learned that maintaining a massive feature matrix was impossible.