> Why does the world need two very similar planes, such as the Boeing 737 and the Airbus A320? If there was only one plane manufacturer

traditional economist answer: without competing market forces, there would be no pressure for innovation? the high efficiency engines that customers have pressed for from the engine manufacturers, and other improvements which resulted in the 737MAX and a320NEO would not exist if there was a single monopoly small airplane. If there was a single monopoly small airplane people would probably be still flying around in a level of technology/fuel efficiency similar to a legacy 737-400.