But one is a transportation system, really just a form of propulsion. People in the 24th century might go their entire lives without ever setting foot on one. The other is a major factor in every person's life. More than a economic system, it's would be one of the corner stones in the society and of the multi-culture that makes up their civilization. Money and currency are not the same things, currency is money, but money isn't automatically currency. Today, a single troy ounce of gold dust will buy you 260 bushels of corn (about seven and a quarter tonnes). That gold dust is money, solely because people will accept it. Money is any object or record of accounting that is generally accepted as payment. 5000 years ago, in the middle east, a shekel of grain was money, it was a standardize amount of a substance (an object) that was generally accepted as payment. There another (or more) piece to that, beyond having a culture where people basically work for free, you have to have a culture that never changes away from working for free, not in decades, not in centuries. A mono-culture of volunteerism. Which is almost an impossibility in a modern society for a protracted period of time. Humans will be in contact with hundreds of alien species, with many thousands of different cultures, lifestyles and ways of doing things. Once the culture that changed to volunteerism, later changes then to "something" else, the world wide society that depended on volunteerism to operate either collapses or it adapts. I'm going to stick around for a while.