It would be interesting to see if suddenly in that case the ChiComs suddenly switch their allegiance to South Korea. They've been cozying up in recent years, and the one think China and South Korea do not want is millions of Nork refugees pouring over their borders.
More like South Korea shifts more towards China. Japan would likely do the same; they have no more love for the North than South Korea has. If either country concludes that they can't depend on the U.S. to be there in the event of North Korean aggression then they will have no other choice.