I have a plan ...
Why retire the orbiters to some museum? Why not send them to Mars?
I am thinking the liquid fuel tank could be refilled in space by a few trips by Russian Soyuzs and then the rocket engines of the shuttle can be fired back up again and away we go to the Red Planet!
I am sure there will be human volunteers and food resupply missions can be sent periodically ...
Sure, this will be a mammoth exercise ... extraordinarily expensive but not technologically impossible.
So, the only reason why this is not happening is because humanity is too busy with 'other' concerns such as wars here and there, developing weapons, more wars here and there, age-old problems of poverty, too many people, too little food, etc.
I wish I lived at a time in the future when the average human being was equivalent in intelligence to Carl Sagan or Richard Feynman. Perhaps, that future will never dawn.