The thing is that at Google protobufs are used everywhere. Like, absolutely everywhere. Think of all the places they could be, and it's way more. All the tooling understands them, code search and go-to-reference works on them everywhere, they are truly transformational on how many different services with many different implementations interact.
Are they perfect? Far from it. But a Go-only implementation misses almost all the value of protobufs. If I was inventing them for Python I could do better (pickle? maybe not) but the whole point is that they aren't language/ecosystem/use-case specific. If this was Rob Pike's frustration then I can't help but feel he missed the point, or this post is a little disingenuous as to the benefits.
I've not seen Gobs used in Google, but I'd imagine an engineer would need to make a very strong case against using protos between services, regardless of if both services are in Go.