In the bad old days, slave ships packed in as many people as possible.
They did this not by having them stand - but by having them lie down, which enabled the decks to be closer together.
While not advocating anything as drastic, it occurs to me that by adapting the Japanese pod hotel method, something close to business comfort could be attained for all pax on long range - and a second deck added without excessive height requirement.
Lots of reasons against - boarding, evacuation, weight and pax acceptance included but it might be worth a designer’s pencil on a slow day...
Fire away! B