There's no ebb and flow, current trends are unprecedented.
And, again, the issue is not raw population numbers but the demographics of an aging population.
E.g. - say there's a country with a total population of 20m, of which 16m are working age and supporting 4m retirees. The total population then halves, to 10m over time.
If that 10m compromises 8m working age and 2m retirees, then you still have 4 working age people supporting every 1 retiree.
If, though, you end up with 5m working age and 5m retirees, then its a 1:1 ratio. Either retirees have to receive dramatically less support, or each individual working age person has to do more to support them.
Actual numbers aside, that's the core issue - not that the population is declining but what it looks like demographically, and what that means for people's economic security.
And we're not talking about a "one off" shift, birth rates are continuing to fall but life expectancy is not seeing such a dramatic change, meaning the demographic shift has become (and is expected to continue to get) worse with each passing generation.
If there are solutions available, nobody seems willing to think about them, so towards the tipping point for societal collapse we go...