Boeing say that the alternative to the Sonic Cruiser is "Project Yellowstone" a Super Efficient aircraft (see Flight today) which will use the materials, systems, aerodynamic and propulsion technology improvements needed to make the SC work to make a conventional speed M0.78 250 seater 20% more efficient than B757/B767-300ER/A330 class aircraft. They say that this is what the airlines want. The problem is that the capital cost of such an aircraft would be high and its efficiency more than off-set by the much lower capital costs of recent-build used B757/767/A330s etc and, without increased speeds as a USP, would be a difficult sell. Same reason that Boeing have generated little enthusiasm for the B744XQLR - the current model does its job so well that it's difficult to justify the price required for the proposed developments. As Boing suggests, take a recent, but now reduced value B744, add blended winglets to give 5% more range, engine and aerodynamic mods to reduce fuel burn, new interior and terminate all ADs and you have a 7,500nm 420 seater whose capital costs allow it to deliver seat-mile costs lower than other type - A380 and proposed "777-400ER" included.