I'm pretty sure that if the population got to be too small, then Tokyo simply wouldn't be able run the entire country, so then non-governed areas would become deindustrialised. That would lead to rural de facto traditionalist populations living without the technology-requiring technocracies restricting the vitality (read, birth-rate) of their communities. And then with a growing population, the need for land to expand into grows and then those non-industrialists would eventually become a force to challenge the industrialists in their ruins.
Everything is contingent on immigration moritoriums, to let nature take its course. And that's probably going to be this crapitalist's meaning of warning of disappearance--he wants to push a dehomogenising immigration "solution" just like his fellow conservatards have done in the U.S.