Empires shift legions all the time - and in the cases you mentioned - legions are still there on their manning modern versions of Hadrian's wall.
America is not an empire and no empire has ever acted the way we do. We have been pulling troops out of Germany and South Korea in order to show our displeasure with them. Rome never acted that way, nor did the British Empire, nor the Soviet Union, nor the People's Republic of Chinese. As such, your unwillingness to acknowledge those basic facts reflects poorly on your argument.