That said, it strikes me that maybe the yen is overvalued already.
Japan, as an economic entity, has a nasty century out before it: the mother of all demographic crises, no growth to build on, the ascent of an ultra-powerful historic cultural rival on its western shore.
Maybe a decline in the yen is justified given the big picture
It sounds like a paragraph you pulled from somewhere else. The yen is the most undervalued of all western currencies, it also along with other asian countries has a huge trade surplus, even china imports alot from japan. Its car manufacturers are the world leaders, it is also a world leader in heavy plant, look around an irish construction site almost all the heavy plant is japanese. Japan still excels in technical knowhow which is unique for a modern western country ( other westwern countries have outsourced this). Why has japan a nasty century before it, if it has a demographic crisis its nothing compared to crisis facing china and india. Japan may not have its own oil source but alot of the world burns oil in japanese engines.