They don't take into account the loss of the 4-6 first class seats that could be fitted where they sit on every flight if you removed the cockpit.
Fair point, but you're assuming you could always sell those extra seats, plus in any event you'd still have to find room for all the hardware that might eventually replace the Human Life forms, so I doubt you'd free up all that non-revenue earning volume currently known as the flight deck......
I guess if you got rid of the cabin crew as well you could save a considerable sum......