That PS4 decline is much bigger than what anyone thought it would be. The fact the PS4 will sell 1 million less than the PS3 in Japan shows that PS is really declining there. There is no way the PS5 goes above 8 million unless something unexpected happens.
But we have to factor in what we're already seeing, which is the PS5 comfortably outselling PS4. Demand for it in international asian markets may be a big part of this, so the consoles may not literally be sitting in Japan but as far as famitsu numbers tell us, the PS5 is outselling the PS4 (ATM). Will be important to see when sales reflect demand and not just supply.
What really needs to be factored in is scalpers.
I've come to the conclusion that 2020 created far more scalpers than we've ever seen before (and worldwide too, not just Japan). We had this perfect storm where brick and mortar retailers had to close while demand for video games was skyrocketing. Also a lot of people were unable to work. That was the perfect climate to create scalpers.
Last year, the Switch would have had shortages even without scalpers, but I do think scalpers made it a lot worse. This year, I think the PS5 is in the same situation. There would be some shortages anyway, but scalpers are making things a lot worse. I expect next year the Switch OLED model will be the device to be heavily supply constrained. Scalpers need to move on to the next hot device. Then in 2023 there will hopefully be no hugely in demand hardware and a lot of these scalpers can die off. Until then, I think we are going to see every shortage magnified as scalpers try to take advantage of any type of shortage or scarcity there might be.