Why to ground Space Shuttles on Earth if they could 'retire' in space?
alt219: Cost.  Well, besides the need to maintain something that was never designed to be in space for a long duration, I would guess it would be because launching a Space Shuttle mission costs roughly 1.5 billion dollars (according to Wikipedia).  And while there would be costs associated with decommissioning a shuttle, it seems likely that it would be significantly less than 1.5 billion dollars.