I worked in cargo for a Latin American airline in the 1970's. Our fleet consisted solely of 727's. Back then, cargo prices were regulated. Our cargo operations paid for the airline's costs; passengers were profit. Even without passengers we paid the bills. Also, keep in mind that the aircraft would pick up passengers in Bolivia who were going to Miami. If you were in Paraguay flying to Bolivia there were a lot of better local choices than waiting for the flight from Miami.