The problem is the typical 1.2 -1.6 white European fertility rates, when the society's median age edges up past menopause. Historically, no known society has every recovered from that. The number of those not in the workforce based on age or disability inexorably outnumbers new workers coming into the worksforce, which puts increasingly big costs onto the potentially childbearing generation. They bear the burden of provide the upkeep for their elders, and thus they keep delaying and deferring their own childbearing.
The new factors here, are the universal distribution of the means to effectively stop childbirths (contraception, sterilizxationa nd abortion) coupled with a much longer life expectancy (~80) prouducing a huge elderly class plus a childless young adult cohort simultaneously.
This did not happen during the Black Death. This did not happen during the Mongol Invasions. A few years ago Italy became the first nation in history where there are more people over the age of 60 than there are under the age of 20. Germany, Greece and Spain as well as Japan and other Asian Rim nations, have now crossed that same divide.
Don't forget that the potential for childbearing is pretty much limited to about ages 15-35 years old for the females in each age cohort. Fertility declines drastically after that. If you have a situation where the females basically aren't coming anywhere near replacing themselves (think Bologna, Italy, where the fertility rate has been under 1 --- more like 0.8 --- for 20 years), you have a pretty near irreversible problem.
As I said, no known society has ever recovered from this. I didn't do the calculations, anyway. Did you check the links?
Sigh. If I believed that, I'd have fourteen children when I reached 52.
Although it's not really relevant to population-wide reproductive outcomes, I believe that the "loss," as it were, of around 15 years of fertile time for women is almost entirely the result of lifestyle choices, not nature. In modern industrial societies, we're actually seeing a longer period between menarchy and menopause than has ever previously been observed. If most women can't have a child at 40, it's often because they spent the previous 25 years trashing their reproductive systems.
I’m just thinking outside the demographic dogma/status quo box.
1. Very likely European peoples will be *forced* (do or die) to make some harsh adjustments. The 50 or so years of the social hammock we’re seeing now will be over soon. Boo hoo. Kicking a heroin habit must be hard, but some people manage.
2. There is no biological reason why women 20, 40 or 60 years down the road will not have 3, 5 or 10 kids. The motivation for that will be greatly enhanced when the encroaching muslim hordes have been kicked back to wherever they came from.
3. Retirement is a social construct that I think is historically recent. There is no biological reason that people do not work until they die or are supported within the family (3-4 generations living under one roof - like it was for centuries). Heck, my two senior partners are both older than 70 and still working every day (not for lack of funds!). They’ll die at their desks. But they love what they’re doing. Of course, this does not apply to all professions. Boo hoo.
4. Yes, the status quo is unsustainable. From that does *not* follow that the European peoples will simply disappear in 100 or 200 or ... years.
5. I don’t subscribe to doomsday scenarios (global warming, the new ice age, demographic meltdown, whatever someone comes up with to - more often than not - further their own profitable agenda...need I mention Al Gore?)
And a resounding YES to your first question! :-)