Switch sales in Japan will overtake the 3DS... and the 3DS even outsold the PS2 in Japan. So yeah, it doesn't really matter what Sony has in store with the PS5, it's not going to come close sales-wise in Japan. Switch will dominate the charts in Japan probably for the next 3 years.
I feel like PS5 needs a very attractive pricepoint, which is almost impossible considering how expensive the PS4 still is.
And it need one absolutely must have Japan-focused launch title. Like a Monster Hunter World 2 or a complete version of FFVII Remake. Which is also something I would categorize as almost impossible.
I hope the PS5 can at least sell as well as the PS4 but it could be a rocky road.