Yeah, if the number of people shrinks, so does the size of a market, and so do revenues of companies. If you own stocks, those will be worth less. If the number of people shrinks, so does the demand for living space, and the value of your apartment goes down. Etc.

Young, productive workers and consumers will exist somewhere globally, it’s a matter of capturing some of that productivity as investment return. If it all goes to hell, sure, the rows in a database are going to mean very little and you’re going to fallback to fuel, food, and firearms until the end (unlikely).

Not all markets are equally accessible by public companies as is the case with the US. Often you have a lot of private companies, companies owned by people close to the government, etc. Also, in coming decades, declining birth rates will be a global concern, not just one of the developed countries plus russia (and OMG does russia have this problem right now).