The space shuttle was not retired because it was too easy to keep them in flight/orbit. The space shuttle as a whole, with the boosters, external tank, wings,

The space shuttle was not retired because it was too easy to keep them in flight/orbit. The space shuttle as a whole, with the boosters, external tank, wings, wheeled landing gear was also very complex and costly. More complex than a SpaceX Falcon 9, dare I say.