Why is our post editor stuck at cols=40?

You can use the following (mu-)plugin to change the height. You just need to adjust the height as you need it.

<?php
defined( 'ABSPATH' ) OR exit;
/**
 * Plugin Name: (#83974) WCM TinyMCE height
 */

add_filter( 'the_editor', 'wpse83974_tinymce_height' );
function wpse83974_tinymce_height( $html )
{
    return str_replace(
         array( 'cols="40"', ' style="height: 360' )
        ,array( 'cols="400"', ' style="height: 3600' )
        ,$html
    );
}