How to make Users sort in reverse order by default?

You can override the default query arguments using users_list_table_query_args:

function wpse_214577_users_list_table_query_args( $args ) {
    if ( empty( $args['orderby'] ) )
        $args['orderby'] = 'ID';
    if ( empty( $args['order'] ) )
        $args['order'] = 'ASC';

    return $args;
}

add_filter( 'users_list_table_query_args', 'wpse_214577_users_list_table_query_args' );