This isn't said in jest, so please don't take it that way, but, if China killed off America, that would be like killing the goose that laid the golden egg. Who would buy their patio furniture?
I've had the same question. However, if you look at things through their eyes, they would have the entire planet at their own disposal then. They only care about "money" right now because a capitalist society is the "hegemon".
Their rulers actually think Marx was right. (Because communism gives the ruling class ultimate power.)
They needed us only to establish their manufacturing base and modern financial and distribution systems. Whether or not this disease cluster was engineered, China will eventually become fully self sufficient and external trading partners will no longer be an impediment to invading Taiwan or the world.