Demography is not as simple: China have chosen in the past an apparently logic solution, limiting numbers of new birth per woman so lower the total population. After a certain amount of time they realize that doing so new humans found MORE elders then their parent, generation after generation, as a result they have to work to care them instead of having a life. As a result the society can't evolve, there are not enough HUMAN young and active resources to evolve.

A war kill many, but mostly young, so again you'll end up with a mean elder population. An epidemic typically kill the elders BUT no one can really program it enough in a mass controlled genocide that only modern sibling of ms adolf from Austria can imaging.

The real problem is that we live to live. Not to work, not be workers. Thief's and criminals against humanity who actually run many world society, essentially all the westerners and most of the third world countries, pretend to treat humans a software, slave workers and can't really foresee the real outcome. They dream a future they can't build, that can't work, and they pretend to impose their vision stating that no, it does work indeed.