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I think the point still stands though, right? The moon is only 1% the mass of the Earth so if we had all that extra crust it wouldn't make our crust to core ratio so off that we couldn't have an atmosphere.

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Read TSiege's correction again, it wasn't a (bad numbers) 1000T planet being impacted by a 10T object, it was a 500T object (earth) being impacted by a 500T object (now also earth) and ejecting a 10T object (now moon) from the debris.

Without the event we'd have a planet even smaller than mars!

I guess the question is rather than having 2 planets of 500T each form then collide, couldn't a single 1000T planet have formed from the same material?

I think the answer is yes, but the only way you get the dynamo of our core(thus strong magnetic field) is that collision, but that's just my fuzzy understanding from occasional reading over the years.

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