(1) in the long run, Nikkei will probably hit its 1990 top and go higher.
(2) the question remains as per the development between 2015 and 2018
(3) here too you can observe that the correlation USDJPY/NIKKEI is not stable.
* in 1985/1990, JPY strength meant strong nikkei
* in recent times, JPY weakness means Strong Nikkei.
(4) Conclusion derived from (3), when an economy is really strong (like the US in 95/00), its currency appreciates at the same time as its Equity index. when an economy is weak, it needs a currency competitive advantage to thrive. This is where we are.