you don't seem to have an explanation for why negative population is a problem. i'm assuming you've read a lot about the burdens of elder care in those nation amid declining populations. those *are* issues of consumption and standard of living.

i'm saying, it's the other side of the coin as the problems created by unchecked population growth. the lack of sustainability is the issue. population projections just give us one limit (and there are other limits, like global warming) on how long we have until sustainability issues catch up to us.

europe (and russia's) population bubbles are small, and the problems associated with what you're describing are going to be solvable. europe's peak is projected to be followed by a leveling off, and the level off is higher than where we are right now.

http://ec.europa.eu/eurostat/statistics-explained/index.php/File:Projected_population_as_of_1_January,_EU-28,_2014–80_(¹)_(2014_%3D_100)_PF15.png

population leveling off in 2 generations is not a crisis. it's something they are planning for and working through.

japan is a different story, but the lesson there one of unsustainable population GROWTH, not the dangers of negative population. population density in japan is 10x that of the u.s. (and the difference was greater than that at the height of the baby boom in both countries). the situation now is a direct result of that. japan actually experienced something akin to the crash that we'd worry about from overpopulation, because the population density and import costs made having larger families economically impossible for so many young japanese.

south korea is facing a similar (but less dramatic) population bubble, and its not nearly the same crisis as it is in japan because the it didn't have as dramatic of an early economic/population explosion. south korea also seems to be slightly more open to re-examining it's national identity in order to deal with the next century than japan is.

if people push the economic growth model in the next 50 years, it's going to be much worse in india and china than it is in japan currently. but it's the irresponsible growth and lack of planning now that's going to do the damage....not the declining fertility rates themselves.

and there are plenty of countries in the developing world (a good chunk of africa for example) that show no signs of slowing down over the next century.

the best thing that can happen to any population (assuming we don't gain access to new resources) over the long hall is for fertility rates to drop sooner rather than later and quicker rather than slower.

you got the "underpopulation" story backwards.