Loop custom taxonomy to get lists of cutom post types?

You can simply use a tax query to get all posts attached to any product_cat term:

$args = array(
    'post_type' => 'product',
    'tax_query' => array(
        array(
            'taxonomy' => 'product_cat',
            'terms' => get_terms( 'product_cat', 'fields=ids' ),
        )
    ),
    'orderby' => 'menu-order',
);