If birth rate plummets and global population start to shrink in the 2030s, what will happen to the stock market?

Just some intellectual discussion, not fear-mongering.

So there was this study https://thehill.com/changing-america/sustainability/climate-change/563497-mit-predicted-society-would-collapse-by-2040/ that models that with the pollution humanity is putting in the environment, global birth rate will be negative for many years til mid-century where the population shrinks by a lot. What would happen at that time and what stock is worth holding onto to a world with less people?