If they wanted a fresh start on our planet they would have to eliminate more than just us. From a bio-mass perspective we are just a drop in the bucket on good old earth. They would have their hands full even after we were gone. From whales to micro-organisms, many animals would likely be lethal to them. Then again, they may already be here?? (sorry, that was a remnant of the woo in me, you know like the borg thing kind of)