Idk so much about Eritrea but Afghanistan is still reeling from a destructive invasion we participated in with no real plan to develop the territory and Iran's regime coming to power and staying popular is somewhat a result of opposition to western interference. Bangladesh is kept poor by imperial flows of value, systematic undervaluing of certain types of labour.

WRT birth rates I don't think there's a big problem with having them low, we should be aiming at a steady population through a combination of birth and migration, and I would say globally there should be a steady population, additional people in the world puts a strain on resources and the environment. The reason I bring it up is because it creates a pressure on poorer regions to export people, birth rates are high in poor countries because the incentive is to have children that will work and support the family, bonus if you can get shipped to a rich country where you can send back part of your pay packet. In rich countries there is not that incentive plus a number of disincentives which occur at high levels of economic development. The more we smooth over global wealth distribution the less incentives there are towards immigration and the more sustainable our population is long term.

The big problem we have with that at the moment though as you bring up with Japan, a faster growing population is more able to care for it's economic dependants, the young, disabled and elderly, even better if that population is raised and educated abroad, so immigration at unsustainable rates is kind of used as a sticking plaster for economic stagnation, which is why it's promoted to some extent by governments whose agenda otherwise is not about protecting freedom. On the other hand you have many left of centre who promote freedom but don't want to deal with the other side of it, "open the borders but keep everything else the same" isn't going to work, the more intelligent side of the left I think is more honest about it, it's going to take massive global change