If China really shrinks by 300,000,000 over the next 50 years, visiting empty cities filled with mummified remains might be commonplace(economies can’t pivot fast enough to handling such a high death rate per active worker).
However industrial societies are very efficient so I don't know what you mean by "can’t pivot fast enough to handling". China could easily handle high death rates per average worker, practically speaking there isn't an upper limit to how many deaths can be handled. Corpses are generally fairly cooperative and theoretically they'll even go away by themselves if left alone. It is caring for old living people that is the hard part.
This is dangerously incorrect, as India is continuing to discover.
All you need to do is not poison 99% of your vulture population, which is what India did. They learned their lesson, and based on your Wikipedia link, the poison is now banned and populations are starting to recover.
It is obviously possible for old people to live too long so as to become a net cost to society, which means the resources used to keep them alive take from resources that could be spent elsewhere (such as on the young). In fact, that is the top political problem in almost all modern democracies, even though it is not stated that plainly.
I wonder how social norms could change if faced with a very large number of such cases. Perhaps people will relax social norms around mourning. Like, eventually becoming acceptable to have unclaimed bodies bagged and left in the curb to be collected by the same truck that collects municipal solid waste, to be landfilled.
We and other animals tend to mourn those that we personally knew and were dear to us. This situation is a bit different, as it involves a surging share of dying people that were left with no close people to mourn them, or even simply notice their absence.
BANG
(People go absolutely bananas about fertility dipping briefly below replacement, as if this isn't a contingent phenomenon that can and will be changed as circumstances and population change)
Why? People have died at relatively faster rates in populations throughout history.
This is old age we're talking about. All of those three hundred million people are going to die anyway over that time period.
> much worse than any wars I can think of
The Taiping rebellion killed somewhere between 20 and 30 million people, approximately 5-10% of the population of China at the time. See if you can spot it on this graph: https://en.wikipedia.org/wiki/Population_history_of_China - it's lost somewhere in the steep vertical climb on the right.
For a part of that period of dramatic population growth, China famously had a (badly enforced) one child policy.
(also, I guarantee you that there's some maniac in the Chinese ministry of defence looking at a prediction of deaths in a nuclear exchange with the United States that runs into the hundreds of millions, and declaring it winnable. There certainly were on both sides of the US-Russia Cold War, fortunately ignored all the time)
Productivity of 10s of new STEM every few years > productivity of 300m of subsistence farmers / low skill workers in informal economy. Having a lot of surplus people helps in some domains of development / consumption, it's a drag in others, i.e. per capita food/energy security. Thinking about PRC being just 20% less crowded during holidays/events is still pretty stressful.
A feature of St Petersburg is the weather. It helped keep the place sanitary when corpses just froze.