They’re not “transferring” to TBC- anniversary realms become TBC realms. They aren’t separate things.
Anniversary realms are not slated to facilitate transfers. They have always been clearly defined as progression realms which turn into TBC, so there really isn’t any reasonable expectation that anniversary realm characters could be cloned to Era. They are effectively TBC realms in waiting.
If someone wants to keep a character at 60 permanently then they should be leveling/playing in Era, not Anniv realms.