The PRC population demographics currently has about 105 males for every 100 females and that ratio has been higher in past years. I have read some commentary suggesting that can contribute to political unrest and one way of bringing down that ratio is a war.
Overall, I’d be very surprised if gender ratio was that close to parity. It’s like 102:100 at birth normally. Boys have higher infant mortality and men die at higher rates than women up to at least 30, so there has to be a slight bias in favor of boys at birth to keep things roughly balanced. The cohort born since the 1 child policy is probably more like 135:100 IIRC. Maybe it was “only” 120:100. Whatever. It’s a huge imbalance.
India outlawed pre-natal gender determination to stop a similar imbalance.
The “problem” with the idea of China using a war to restore the gender balance is that they don’t have enough boys to spare. They counted on the current fighting age generation to power the spending necessary to maintain their economy. If they wipe enough of them out to get gender balance back to parity they’ll be be even worse off economically than now. Assuming a fast war which they won they’d still have most of their elderly population but they wouldn’t have enough consumers/producers to afford the costs of such an elderly population.
In addition, if they get belligerent all it would take is for Japan, or Australia, or India, to put a couple destroyers or subs or even frigates/cruisers in the Indian Ocean to stop the flow of oil and coal into China. They import 85% of their energy needs, mostly oil from the Persian gulf plus coal from Indonesia, Australia (at least pre-Covid) and Russia. Within a month or two at most they’d lose their ability to project any kind of military power and within 6 months they’d cease to function as a nation on any level, likely losing a third to half of their population to famine within a year. That would solve their aging population issue pretty thoroughly, but would also leave them a fractured nation incapable of participating in a global economy for years if not decades.