to a point where the health problems it causes become seen as a public health problem, is a gradual shift in population towards the kansai, precipitated by government and corporate relocations.

moving a 30 million megalopolis is not going to be quick. building the housing, infrastructure, and commercial networks necessary to sustain that number of people cannot be built overnight, even were everyone to try to flee west. the roads, train lines, etc. alone would prove to be a bottleneck.