There is some logic to your maths, but if you travel near to the front then cross-platform applies to 100% of the Up trains. Even if not, it's a quicker transfer than most train to tube ones. And headed Down, agreed. But would doubled frequencies not mitigate some of this time in terms of netting out over the course of a travelling week. Mornings yes you leave in time for a specific train, but evenings you may not - or may find yourself procrastinating at your desk until it's time for the next train. At a higher frequency, you might gain back more personal time.