I don't understand why this is a problem. We are moving through demographic transition, as the world reaches its carrying capacity for human populations. So, we are safe from overpopulation crisis, from future unemployment crises, etc.
This is compounded by what was (until recently) a steady increase in life expectancy.
The really frustrating aspect of this is pensioners, who have traditionally huge voter turnouts during elections, are unlikely to have their benefits cut, because it would be political suicide for a party. The elderly believe they have "paid in" to their plans and deserve them, but outside of private pensions, this is usually a significant over-simplification. Instead, what will happen is the working young will need to pay for the generous promised benefits that the elderly are receiving, whilst 'paying in' for much lower benefits for themselves.
So the future with less workers is just a future with poorer billionaires and they don't like that which explains why all the media owned by them are trying to scare us about againg population scenario.
Pension system in most world assumed steady supply of productive populace when it was set up. (=assumes certain young / old people ratio).
A lot of asset backed valuation (real estate etc) assumes certain consumption level, demand, and steady grow. So does inflation / investment / wealth management.
Healthcare systems (especially socialized) also have certain expectation on the distribution
If we just equally reduced population in the demographic distribution (same amount of % of men/women young/old) things wouldn't be as much of a problem.
But as it is, most countries would struggle with the transition. That's why it is a "problem" people's talk about; what is the least stressful transition strategy?
I don't think the solution should be to prop birth rates back up, necessarily, but we do need to consider how to deal with the economic consequences of this demographic transition, and I don't think any mainstream politician is currently willing to deal with a world that doesn't experience constant population growth.
1. Fewer productive taxpayers to pay for old-age pensions
2. A drop-off in available investment capital as private pensions are converted to bonds and cash
3. Fewer workers for an economy with soaring demand for specific fields e.g. medical