If the reserves you mean are in the form of money, this will not always save our lives in old age, because the value of money can also be eroded by inflation or by increases in the price of goods every year. So the only thing that can help our lives in old age is investing in assets whose value will not be eroded by inflation or any other conditions.
Saving in fiats is also important because it can be used instantly. Meanwhile investment, it sometimes takes time to convert it to fiats, it cannot be used instantly. So, you can't state that only investment that can help our future. You must understand both saving and investment are very helpful for our future. Don't only view it as something that can multiply our money, but also view its function in our daily life! We do any transactions in fiat or our national currency, we don't use investment for this purpose.
And if you see an old person who is starting to have difficulties in his old age, even though he used to be rich in his youth, it means that he did not invest his money so that his old age becomes more difficult.
People have different situation. Even if there are people who experience it, it doesn't always mean they didn't try to have investment or saving in his youth. They may already use all their saving or investment for a certain purpose. We don't know what the experience of each person, people sometimes have financial problems due to certain problem in their life. So, don't judge people easily!!