Think of salary structures. Most Long Haul operators promote pilots from their Short/Medium Haul Ops. Would SH/MH folk take a big drop in earnings to work as a back-up cruise pilot on the LH fleet? Obviously not. I cannot see where the airline's cost savings would come from given the salary structures of SH and LH being roughly comparable. A well-paid SFO on SH is far more likely to want to move to the left seat on SH than step way down in earnings to right-seat/sleeper seat on LH. And how would the new and lowly-paid FO/SFO in Long Haul ever get trained properly? While they are asleep? And does the Captain stay awake for the whole flight?