Standing passengers? Not yet they don't, and I don't see even the authorities standing (sorry) for it. Apart from anything else, what about the vertical G forces that a seat has to withstand? The other problem is that the aircraft in its present form can only carry 189 because of the emergency exits. More pax would require re-certification, something I don't think Boeing would be happy to do. Standing pax = bad publicity. MO'L might consider bad publicity is better than no publicity, but I suspect Boeing will think differently.