DVT Express.
Sitting down for 17+ hours is quite a challenge (don't forget they've probably also been sitting down for over an hour at the terminal) - will everyone be getting up for a leg stretch?
Besides the issue of the toilets clogging, becoming occupied for long periods of time, running out of water (has happened to us a couple of times on LH), all it will take is some expert to say that sitting down for so long is considered unhealthy and the opportunists among the pax will follow through with law suits for compensation (i.e free trip).
Can't stand LH flights as I get older unless the plane is half empty, but what an expensive luxury that is.