I might have been a bit too broad in stating that the turbojet is the best at what it does, but "speed" is a design characteristic just like any other and alter

I might have been a bit too broad in stating that the turbojet is the best at what it does, but "speed" is a design characteristic just like any other and alternatives will need to come close on the speed side or else anything they may provide on the economy side of the equation will be ignored. Turbojets fly as fast as is practical given the market demand they are serving. If airlines offered people the choice of flying half as fast for a lower price the fast flight would still win (up to a certain point, as the supersonic option showed when Concorde was still flying.) You could do even better by putting your turbofan onto a rigid airship, but that isn't going to happen either :)