I've always wondered why airlines don't have stacked bunks on longhaul routes, as an option, rather like the couchettes that you get on trains but closer together, for space economy, not comfort. Without analysing it, I would have thought that you would get more people lying flat onto a given space than seated, even in densely configured economy class.
There is wasted space above seated passengers' heads, and if the 'bunk' were closer to the cabin floor, you could get two or three levels of bunks.
The trade off would be that although they would be small and narrow bunks, not even big enough for any airborne hanky-panky, some passengers might prefer lying down. I appreciate that it might not suit everyone, there would be limited possibilities for IFE, eating and drinking but I think it could work.
Emergency evacuation might be a barrier to this.