Fifteen hours sitting straight up sounds pretty painful...I honestly don't know who *could* do that, let alone who would.
I did AKL-LAX , 12 hrs (after commuting up from NPL) and then, after marching round LAX in Immigration and Customs queues without rest, got stuck in a non-reclining seat (in front of exit row, last seat left, almost Christmas) for the next leg to LHR, before getting my last leg to ABZ.
I got sciatica like you wouldn't believe on the AA leg and there was no respite at all.
To those who sincerely believe that "there is no comfort gained" by reclining your seat, why do you think people do it? Do you think they hit the recline button accidentally?