No other nation, and certainly not the US, can and will sustain millions of casualties in battle. China (at least its totalitarian leaders) would not only be willing, but eager to do just that if it restores “Greater China.”
One other consideration is the fact that every man Jack of those "excess" 30 million men is an only child. Whose parents will be less than thrilled at the thought of those casualties - and have the sympathy of the rest of the population of parents of only children.
Your point is valid, but I’m afraid parental anguish doesn’t sway totalitarian regimes.