Back in the early 90's the Chinese railway system had some cars set up exactly like this. They were called "hard sleepers" and they had a vinyl cover over a bi

Back in the early 90's the Chinese railway system had some cars set up exactly like this. They were called "hard sleepers" and they had a vinyl cover over a bit of padding with a narrowish (i'm guessing) plywood base. They looked not promising but they were surprisingly comfortable for even two or three day rides. You could either sit or lie down on them. A modern airliner probably wouldn't go for plywood but if the safety aspects could be worked out the general design would be a big improvement for long haul flights.