The sitting position occupies a cubic volume with dead space below the butt and above the lap. If you want to put more people in the plane, make them lie down and place the lie down spaces on top of each other and side by side. I'd be down to fly in an airplane like that which by logic should have cheaper air fares because of more people on the plane.

It's the perfect sleeping position. The problem is Would you rather be lying down all the time or sitting down all the time? Overall I prefer lying down because I have the option of sleeping and staying awake while lying down; while when sitting, I have the option of staying awake and a really hard time getting some shut eye.