One feature of Boeing in recent years is how the whole catalogue of the latest developments and stretches have been such poor sellers. 717, 737-900, 747ER, 757-300, 767-400, 777-300, all have sold way below expectations, while the older models they were derived from (717 apart) continue to sell much better. This has never really happened before. Boeing R&D just isn't developing what the airlines seem to want.