In round numbers, let's say out of 1.5 billion that 500 billion were gone via nuclear retaliation. That 1/3 gone represents the majority of the economic and political capability of the nation. I don't think this assumption is far fetched.
Would this nation cease to exist as a coherent entity?
Didn’t Russia lose 20-30 million people during WW2? That’s probably 15% of their total population. The Chinese are hardwired differently than the people who live in the West. I believe their tolerance for a disaster is much greater, I would say it would take 1/2 of the population disappearing before the CCP gets overthrown.