In the hypothetical situation where Tokyo needed to be evacuated, or a large enough portion of the population there feared for their health and wanted to leave, i wonder if in the long term, the redistribution of population back into regional Japan would not be a good thing over all.
People have a hard on for regional areas, but the practical reality is people enjoy the benefits of living in cities too much to want to live in rural areas. They are also way worse at actually providing for the needs of those people efficiently.