The most fundamental is that within a few years existing and new issue government debt may absorb all private savings (corporate and household) in Japan, making foreign borrowing inevitable and creatinga terrifying scenario of higher yields demanded by foreigners raising MOF interest costs while inflicting portfolio losses on JGB holders, and putting fiscal balance further (infinitely?) beyond reach, with (by then) predictably horrendous consequences.
It allows the markets the ability to experience shallow pullbacks, creatinga perfect technical picture, while the fundamental scenario on both a micro and macro level continues to improve.