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Changing a button label of WP_Customize_Image_Control() doesn't take effect

I tried to modify the Customize Image Control class and replace the instance of 'default' => __( 'Default' ) with 'default' => __( 'NEW LABEL' ) and re-classname it like this:

class WP_Label_Image_Control extends WP_Customize_Upload_Control {
    public $type = 'image';
    public $mime_type = 'image';

    /**
     * Constructor.
     *
     * @since 3.4.0
     * @uses WP_Customize_Upload_Control::__construct()
     *
     * @param WP_Customize_Manager $manager
     * @param string $id
     * @param array  $args
     */
    public function __construct( $manager, $id, $args = array() ) {
        parent::__construct( $manager, $id, $args );

        $this->button_labels = array(
            'select'       => __( 'Select Image' ),
            'change'       => __( 'Change Image' ),
            'remove'       => __( 'Remove' ),
            'default'      => __( 'NEW LABEL' ),
            'placeholder'  => __( 'No image selected' ),
            'frame_title'  => __( 'Select Image' ),
            'frame_button' => __( 'Choose Image' ),
        );
    }

    /**
     * @since 3.4.2
     * @deprecated 4.1.0
     */
    public function prepare_control() {}

    /**
     * @since 3.4.0
     * @deprecated 4.1.0
     *
     * @param string $id
     * @param string $label
     * @param mixed $callback
     */
    public function add_tab( $id, $label, $callback ) {}

    /**
     * @since 3.4.0
     * @deprecated 4.1.0
     *
     * @param string $id
     */
    public function remove_tab( $id ) {}

    /**
     * @since 3.4.0
     * @deprecated 4.1.0
     *
     * @param string $url
     * @param string $thumbnail_url
     */
    public function print_tab_image( $url, $thumbnail_url = null ) {}
}

And later use it like this but the Default button when "No image selected" still have a 'Default' label instead of 'NEW LABEL'. Anyone who can fix this?

$wp_customize->add_control(
        new WP_Label_Image_Control(
                $wp_customize,
                'logo',
                array(
                    'label'       => __( 'Label', 'theme-slug' ),
                    'description' => 'Description here..',
                    'section'     => 'section_id',
                    'settings'    => 'setting_id',
                    )
                )
        );