The fact that they will have many more men than women in the next few decades should scare everyone.
Collapsing the global economy is not conducive to achieving this goal. People all over the place will tend to get snippy about it.
They are involved in a different type of warfare.
So were the Japanese. They lost. People who choose to treat economics as a form of warfare don't have a clue as to how to do well at either.
Even if the war does get hot, much of China is already just a step from the stone age.
That part of China is completely irrelevant to China as a world power. The part that's urbanized and industrialized is the important part--and without the ability to transport food from grower to consumer, that part of China will fall apart quite rapidly.
They would be far less affected than the US.
They wouldn't be anything resembling a nation-state, either--just a bunch of people seeking to eat their neighbors before their neighbors eat them.
The fact that they will have many more men than women in the next few decades should scare everyone
Not everyone--mostly, it should scare the people living in China now who will be around for the result. In theory, that means that there's a huge surplus of men available for soldiering.
In practice, considering how xenophobic Asian cultures are, it means that there's a huge body of men with ZERO prospect of being co-opted into helping perpetuate Chinese society, because they will not have brides suited to Chinese cultural sensitivities. And those young men will know this.
In that kind of environment...
Giving those young men military training--assuming that Chinese society can hold together under the stresses of millions of Chinese juvenile delinquents, which is questionable to begin with--is somewhat akin to playing with a fueled-up flamethrower in a powder magazine. The results are predictable...and f'ugly.
China's going to be a first-order example of what happens when you combine Enron-style accounting and near-genocidal social engineering practices. Get yourself a soda and some popcorn, boy, it's going to be quite a show when the wheels come off.