gRPC actually does have easily identifiable URLs. Protobufs can be heavyweight, but it's trivial to use a different encoding like JSON.
These all qualify as "more heavyweight," at least from an implementation perspective.
That shouldn't be necessary. Protobuf changes are supposed to be additions-only, where existing field numbers never change their meaning, but new optional/default fields are added. If you do that, you only need one version of the proto spec.