The true reason for the Japanese decline? Plaza Accords.
With that, America not only disembowelled Japanese enterprise and their economy, it also neutered their national spirit. Before that the Japanese were on the ascendency and basically beating America at everything they focused on, with the same work ethnic but no population collapse.
The Japanese nation today have no fire, direction or drive left. How could you when your master will come and chop your balls off as soon as you show any signs of growing a pair? Without that is it any wonder they are stagnating? Their entire nation is basically in a collective mental state of deep depression, hence a lack of will to even make babies or even to continue living, as the alarming numbers if their suicide rate shows.
Their entire society has been reshaped into a perfect subservient white worshipping servant culture. The only time they seem to show any spirit or passion is when they are trying to kick down their fellow Asians like Koreas, Chinese and Philippinos etc to maintain their top pet position with their American masters.
That’s the core reason why their economy and demographics are circling the toilet with no signs of recovery.