Well what does the PS5 have in Japan compared to PS4 other than the backwards compability?
Price is higher
Launch lineup is mediocre from a Japanese perspective and the games are going to cost even more.
Console is too big in size.
Sony made it more and more clear that Japan is not their focus.
PS5 will have a somewhat strong launch period but once that is over it has little going for it so I expect it to drop quite low after New Years.
Don't get me wrong, I'm expecting the same thing. But if the PS flubs the launch to the point to have the same sales in November as it's predecessor had in late February, then it will have a hard time even compared to the Wii U, at least for the first couple years.
The Nintendo eShop rating Thread: http://gamrconnect.vgchartz.com/thread.php?id=237454