What make you think we will have better propulsion technology in 20 years? I has been 33 years since Apollo 11 went to the moon and we have mode only very minor improvments to space propulsion systems. Most of these improvements were made as part of the Space Shuttle system in the 1970's. In essence, space propulsion today is not any more sophisticated than space propulsion was in the late 1960s.
Time alone does not cause technogy to advance. An active effort is needed. From 1961 (Kennedy commits U.S. to the moon) to 1967 (first unmanned Saturn V launch) the U.S. advanced the state of the art on space propulsion. That would not have happened if not for the money or pressure of Project Apollo.
That could be said. However, rocket motors are more efficient, and deep space propulsion is in a new generation. They won't need Saturn V to go to the moon anymore.