Sorry, I'm not sure I understand where we are disagreeing.
An existing package either has to freeze all existing public enums explicitly, or break API, or not adopt the feature (or any new language mode incorporating the feature)—that is the complete set of possible options regardless of what other bells and whistles are added to roll out this feature, no? As soon as we commit to this as the plan for the language, there is no escape from making one of these choices for existing package maintainers.