Think about the old-age ratio.
The old-age ratio is the number of not working/economically inactive compared to the number of people of working age.
The older generations (financially inactive) are dependent on the working group (let's say ages 15-64), since they pay the majority of the taxes and have essential jobs. A lot of elders are dependent on financial aids (which are paid for by the tax-payers) and essential workers (nurses, doctors etc.).
If the younger generations were to disappear, the working group would get smaller and smaller as more people retire. As the working group gets smaller, the retired group gets larger, creating an increased demand on the working group until it would eventually reach a breaking point. The old-age ratio would increase.
