I can't follow that argument, as it seems to apply just as much to wanting to change the type of a field, the name of a field or just making changes in general. Maybe I deal with relational databases too much, but I really miss being able to declare fields required and provide default values, because now instead I have to document, trust others will read the documentation, and provide explicit error handling for when constraints that used to be validated client side or server side by protobuf. I also miss not having a NULL value, but protobuf never had that and almost certainly due to dealing with too many relational databases ;)

If I lose oneof too, I might as well be using JSON.