This is pretty much verbatim what I've been saying. One thing thats interesting about the herbivore crisis is that the Japanese government much like the left in the west are aware of the crisis of low fertility rates, but their only solution is increased immigration which is ironic since the reason why immigrants from the third world, particularly Islamic countries, have such a high fertility rate is because their women have severely limited rights.
It might also be because they are more focused on wanting to marry young and have as many kids as they can, whether they are poor or not. A lot of Americans (especially the middle class) think they can't afford 1 kid when the husband is pulling in $40,000 a year. 'Idiocracy' showed this perfectly. Poorer Americans are not as careful at planning but often use abortion. Many Muslims will not use abortion. They have faith that God will take care of things. Some Christians still believe this, but fewer than in the past.
Limited rights is a factor, but I think materialism is a bigger factor (they often go hand in hand). As people get wealthier, birth rates tend to fall and this is true even in many Islamic nations. It is actually the opposite of what one would expect would happen. As health care improves, pregnancy is less risky and more children survive birth. More money means people can feed more kids. However, birth rates fall because people demand more.
Immigration doesn't seem like it is needed though. Japan has a high population density. I don't get why people would be worse off if the population dropped 80%. It would mean housing and food prices would drop, people wouldn't need to be pushed onto subways in Tokyo, and Japan could become a food exporter. I have never been on a subway or airplane and felt that I would be happier if only it were even more crowded.
Crowded cities (made worse by mass immigration) tend to reduce birth rates as well. I read that in cities, children are considered a burden as they take resources but they don't provide any benefit (especially in countries with strict child labor laws). In farms, the kids still need to eat, but they also help on the farm. Mass immigration may also reduce birth rates among the native population and make native men feel even more hopeless.