For instance is it problematic to just import Africans to Japan until there are no more ethnic Japanese left?
It seems like what you are proposing would just lead to the most developed nation's peoples just dieing off and being replaced with groups that are slowest to educate and provide a high standard of living for their women, which would be awful for human cultural diversity in the long term
Whatever you may believe to be a result primarily of culture, wether it be education, treatment of women or what have you, it is much more correlated to development status. Preindustrial Europe/Japan looked a lot more like Africa in those aspects does today than their modern status would seem to indicate.
Edit: also, where did I indicate we should get rid of ALL Japanese people? What a ridiculous straw man.
Why in those countries, when countries in the west that have already completed demographic transition aren’t even at that level and have policies that actively resist fertility rates that low.
Why not encourage development in Africa and west Asia, immigration in to western countries, and family planning in places where headwinds are much weaker (and where a shrinking labor force can be bolstered by immigration).