Well, when I'm traveling for work, I simply don't have the option to stay an extra day on some halfway destination. Therefore, even though a 19-hour flight is painful while it's taking place, it's far less painful than two 9,5hrs flights, with the rigmarole of twice the security, twice the check ins, layovers for hours the stress of missing connections and going through passport controls etc.
I'm sure that a direct London to Sydney route would do smashing business. It's better to just get there in the shortest possible time and not screw around with stopovers.
I think Tartare is right - ultra long haul is the future.