That info is a bit outdated. All but the oldest APIs (most notably GCE) support gRPC out of the box.
For newer services, there is an automatic 1:1 mapping between the REST and gRPC APIs, except for features like streaming RPCs which have no REST equivalent.
The argument that gRPC is disposable because everything is stubby internally applies equally to REST. And I don't think anyone is arguing that REST is disposable.
I'm not sure what part of GCP you work in, but in my experience, the big customers are using gRPC, not REST.
For newer services, there is an automatic 1:1 mapping between the REST and gRPC APIs, except for features like streaming RPCs which have no REST equivalent.