True, but if you're an interstellar civilization prepared to wipe out some primitives, it's probably still cheaper than a military campaign and doesn't risk retaliation or attempts to intercept your missiles.
Plus if another civilization asks "did you just glass those guys!?" You can just say "nope they got hit by a rock, not my fault"
I mean, probably the easiest way to wipe Earth clean would be to just park your giant-ass alien space fleet directly in the path of the Earth's sun, and just follow the planet around on its orbit, blocking enough sunlight to slowly freeze the planet. Then once all the undesirable life has been eliminated, you just let Earth warm up again and claim it.
Even if we knew what was happening, we're not advanced enough or cooperative enough to actually fight an attack like that.
I got curious as to whether the atmosphere would recover from that, and found some pretty cool (sorry) articles describing the sun simply disappearing. It seems like the surface gets cold-in-a-physics-sense in just a few weeks, and the atmosphere, well... condenses/freezes. Which I think mean it *should* recover when the sunlight is restored, though I shudder to imagine what happens when you expose frozen nitrogen seas to direct sunlight. Gravity still works fine, but I wouldn't be surprised if some material gets ejected anyway.
I don't think any of this would penetrate much beneath the surface. Mile after mile of dense rock insulates our molten core... I'm sure volcanoes would stay active. Warm, but airless. Ooh, I wonder how long it would take for the ocean depths to freeze? Maybe some creatures could survive indefinitely by the ocean-floor heat vents? They wouldn't be getting any biomatter from above anymore, though.
...What would a frozen ocean even look like? That's a lot of expanding ice. Seems to me like it'd be a sequence of the surface freezing, then inevitably cracking
under above the force of the lower ice trying to expand against the incompressible seawater. Causing massive high-pressure geysers? That water eventually refreezes... and like a lake, this layer of ice would insulate the ocean quite a bit. But inevitably more of the seawater freezes and pressure builds up again, shattering the surface ice.
And/or the ice expands up over the land, like some sort of glacier in some sort of ice age. But SO much more so. The land would be reshaped, presumably all of it, every human construction being ground under the walls of ice. I wonder how fast such a glacier could go under such conditions?
So basically to survive this, you'd just need:
Sufficient nuclear or geothermal energy
Enough plants to process your CO2 and keep you fed, lit by electric lamps
A deep enough vault to avoid getting evicted by a glacier, but possibly close enough that you can go get some ice (they'll stop... I have no idea when)
...sorry all, I've been exercising and hanging out with my mom lately, and it's made me a little manic I think. Very... fortunate that I don't have any real alcohol at hand.