[AlexanderHowl]

An intelligent civilization could decide to evacuate the Earth because it decides that it needs to repair the biosphere of the Earth so that it could have the Earth as a biological reserve (a civilization that does not have a biological reserve likely does not survive for long as a biological species). It would leave a caretaker population, a couple hundred million people or so, to manage things while the rest of the human population spreads out to the stars. Of course, a sensible civilization would terraform multiple worlds outside its home star system so that it could have multiple biological reserves, though the caretaker populations on the terraformed worlds would be there to maintain stability rather than to repair damage.