There has been no shortage of speculations, literary, cinematic, philosophic, and scientific on the topic of an alien visitation of Earth. I'd like to add mine. I am inspired by a recent episode of the Science for the People podcast, discussing the environmental interactions of populations, and how certain "keystone" populations have wide-ranging effects on other populations.
Sea otters eat sea urchins. Urchins eat kelp. Kelp forests are critical for many species of fish and marine invertebrates. When sea otters are removed from a habitat, urchins proliferate, destroy the kelp, and many other species, dependent on the kelp, suffer. Restore the sea urchins, the kelp recovers, and the other species recover as well.
Remove apex predators and grazers proliferate and remove all the grass, with disastrous consequences. Restore the predators, they control the grazers, and the grasslands recover.
So let's say a race of benevolent space aliens visit the Earth and want to restore Earth's habitat to healthy balance. We humans, if we are enlightened, try to restore the balance and health to polluted lakes, or coastal waters, or grasslands. We want to re-introduce wolves to North American forests to restore the predator/grazer balance. But although we may recognize that our own numbers are out of control, we fail to see ourselves as a keystone species whose overpopulation is driving a global imbalance. Or if we do see ourselves in this light, our "solution" is to try to make birth control available. Out scientists develop ways to make agriculture more efficient we we can feed our numbers, and invent medicines to control disease.
Although we sometimes see ourselves as apex predators, eating domestic cows and pigs and chickens, we are really grazers consuming everything that grows upon the earth, both animal and vegetable. Our predators are diseases, and we have effectively eliminated most of them. Our numbers have thrown the entire ecosystem out of balance and our indiscriminate use of fossil fuels is threatening catastrophic climate change.
A benevolent race of space aliens would see us as we see other animals: as a species that must be controlled to protect the existence of our species. Just as we introduce wolves to the forest or lions to the savanna to keep the grazers in check and insure the survival of both, these space aliens would most likely introduce diseases to Earth, not to wipe us out (I am positing benevolent aliens, not aggressive ones) but to bring our numbers back to a sustainable level. We would rail against them as cruel, but they'd be no more cruel than we are when we re-introduce wolves to a region to kill some of the deer.
I'm not advocating killing 9/10 of the people on Earth, because that would be inhumane. But it's probably what's needed for the human race to survive. Space aliens aren't coming. (They're too far away, space is too big, travel is too constrained.) Whether this is a good thing or a bad thing depends on whether you're more concerned for the welfare of the people alive today, or more concerned for the long-term survival of the human race. I guess I'm in the former category by dint of being one of the people alive today, myself.