Money isn't the issue in the end, the issue is that you still need enough people in working age so that you can use the money to get the goods and services that you need as a retiree.

If a large number of people retire without finding any young successor, services just become more and more inaccessible. Like e.g. if there are not enough doctors in your area because many of them retired, you won't get treatment even with money, because the capacity just isn't enough anymore to treat everybody. If a car shop closes down because the mechanic retires, you cannot get your car repaired there any longer, even with a lot of money, and so on..

An old society means that the area decays, more and more places close down or cannot get maintained any longer. And money doesn't help because numbers in a bank account don't do the real work. There are already a few old regions around the globe where this can be seen