Imagine a future where there are half as many the workers as retirees. It doesn't matter how much money retirees have in their pensions if they cannot hire workers to look after them (do their healthcare, produce the food they eat etc). You'd just get rampant inflation as the richer retirees competed to get workers, and everyone else goes without. In other words, in that scenario the value of the saved money would be eroded by inflation.

Now I will admit that this is complicated by there being other countries, or if we invent humanoid healthcare robots or whatever. But there's a principle involved that someone (/ thing) has to do the work.