If the ServiceTypes are phased out the desktop files which contain the property-definitions should also be phased out.
Except for the KCMs there is no known case where we need to have both desktop files and JSON metadata available. KPackage can also handle JSON metadata for the appstream generation & all other functionalities.
Consequently we should just the metadata in the JSON format in version control. When converting the files one should check if the properties are really needed (see T14483) and clean them up if not. When making a merge requests the translators should be pinged so they can ignore the file changes (the actual strings should not change).