Originally Posted by Lusankya
From what I understand, the Chinese government was facing enormous internal resistance and rebellions (see: Opium Wars, Taiping Rebellion) by the late 1700s, so they may not have been able to conquer Africa in the 1800s in the first place. Earlier than that, I dunno. Maybe they didn't have the technology. I also think that the Americas were by far the more valuable colonization target than Africa, so Europe would still be a major world power.
I don't actually remember why they closed off all explorations yeah, as my historical knowledge during those periods are nonexistent at best. I think it's a decent point to say that Europeans would still have had a decent chance to try to find the route to China and stumble upon and commit biological genocide on the Americans anyway. Then again, it is also possible that China would have searched east past Japan as well, assuming they continued with explorations and the like. In addition there's also the possibility that having the entirety of Africa would obviously have made it possible for merchants from Europe to dodge the Ottoman Empire anyway, making it less likely that the Europeans would have tried to look for that other route as well.
Coming to think of it, this is one of the solutions that would allow the Americans to survive for a longer period of time. No clue if they would still be wiped out by smallpox and the rest of them though.