The PS4 got most of the big third party games and had a lot going for it. But it still failed to reach the 10mil mark.
Now many franchises that previously only released on PS in Japan and/or sold the most on PS are not only releasing on Switch day one but the Switch version almost always outsells the PS version.
Starting with the PS3 Playstation was in a decline with many franchises that pushed the PS1 and PS2 to it's hights also declining in Japan, like Final Fantasy and Gran Turismo. Since then other previously big franchises on Playstation have also declined like Resident Evil and Kingdom Hearts.
With the Switch dominating the small to mid sized third parties and having almost all multi million selling games in Japan for itself the PS5 was never going to sell as much as or more than the PS4.
The low software sales of the PS5 don't paint a positive picture for the future of Playstation in Japan.
Final Fantasy XVI next year will show us how much a game can actually sell on the system, but it will likely end up as the first FF in ages to sell below 1mil lifetime.
Most of the PS software sales are happening on the PS4 right now.
I expect the PS5 to still reach a decent number of sold consoles, like 6 or 7mil, but until WW production ramps up a lot possibly over half of them are being resold outside of Japan.
The big AAA third party games aren't enough to push a console to actually high numbers in Japan. By far the most games that sell over 100k are also on Switch or only on Switch.