“An aging population doesn’t work or save.”
You are describing problems that arise when there are oscillations in birth rates.
For example, you can have a lot of old people with few young to run the economy that existed when the old people were young.
A balanced population will do well regardless of its size.
Balanced populations are steady state. Growing or shrinking ones are unbalanced.
If you have a relative few young people running the economy, their productivity will not support the needs of the population as a whole. Taxes, consumption, capital formation and investment all out of whack.