I was thinking that too, like the Chinese took their burgeoning space program and went somewhere else, but that still leaves everyone else from that half of the world unaccounted for. Maybe the whole "Earth-that-was" business was actually a pan-Asian prank to get all the Occidentals to leave. Gotta have elbow room...

I'm normally not picky about this sort of thing, but I become slightly more sensitive to it in SF, and absolutely more if you're trying to present an entire future world (as Joss does). Orson Scott Card once wrote that to overlook the racial question in SF worldbuilding is to state "I have seen the future, and you're not there."