That is not what is being called a crisis.
Fertility rate being *far* below replacement (1.3 vs 2.1) is being called a crisis.
If the population falls to quickly the ratio of young to old gets unbalanced and there are not enough people young enough to do the work needed to support the population too old to work. All the products and services like food and healthcare that the old consume still need to be physically produced regardless of nominal price, so simply saying "save more money for retirement" is not a solution.