The reason (if I remember correctly) for `get_post_format_string()` fallback was so that if a child theme registers a new format that isn't included in the `_s_post_format_label()` `$labels` array, it would return the format name instead of nothing (undefined index). We could add the remaining formats to the array and cover all the basis.
```
$labels = array(
    'aside' => _x( 'Asides', 'post-format-archive-label', '_s' ),
    'link'  => _x( 'Links', 'post-format-archive-label', '_s' ),
    'image' => _x( 'Images', 'post-format-archive-label', '_s' ),
    'quote' => _x( 'Quotes', 'post-format-archive-label', '_s' ),
    'video' => _x( 'Videos', 'post-format-archive-label', '_s' ),
    
    'gallery' => _x( 'Galleries', 'post-format-archive-label', '_s' ),
    'status'  => _x( 'Statuses', 'post-format-archive-label', '_s' ),
    'audio'   => _x( 'Audio', 'post-format-archive-label', '_s' ),
    'chat'    => _x( 'Conversations', 'post-format-archive-label', '_s' ),
);
```