It only makes sense if the backup cities are already owned by the government. If populating those cities post-disaster requires government appropriation of an entire city's worth of private real estate assets, the resulting financial and political instability could well dwarf anything wrought by the original disaster.
I'm not sure I understand this comment enough to agree or disagree with it. The land etc which is Ordos today was appropriated by the state and they built a city on it. It neither destabilized nor affected the operation of the current government of China in the slightest as far as I can tell. This is the same system that decided to flood several cities so that they could build a giant dam. And while their were complaints, it really didn't do the residents much good then either. So I can't imagine a scenario in China where anything the Government chose to do, especially with the reallocation by fiat, of parts of the country to a different purpose, would result in any political instability whatsoever.
So you think that the appropriation of unused real estate is worse than whatever disaster led to the mass exodus of a population from a major city. Interesting, but that is not how I think.