Considering the PS4 is gonna fall short despite getting even better Japanese support than the PS3 I fully expect the PS5 to decline further and end up at 8-9 million in Japan. Since it's gonna launch with the Switch at its peak and presumably be even more expensive than the PS4 I also expect it to have an even slower start than the PS4 did there.

Man, the state of PS4 sales in Japan is just sad.
It's not Xbox levels of pathetic... But the fact that I feel the need to bring up the Xbox numbers to compare is sad just in itself.
And this is a system had more software support (Mainline titles for Kingdom Hearts, Monster Hunter, Dragon Quest weren't on the PS3) and less competition in Japanese console market than its predecessor.
Really makes me worry about how the PS5 will perform over there.

Yeah PS4 has just cratered in Japan at this point, which means we're almost certainly looking at a constant decline in total sales there from PS2 to PS3 to PS4.

Unless they can pull a rabbit out of their hat with a breakout new IP or something, I expect PS5 to continue this downward trend.

The PS4 got its tail end cut short by the Switch.  For Generation 8 the Japanese chose either the 3DS or PS4 for their main system depending on whether they wanted a handheld or home system.  For Generation 9, almost all of Japan is choosing the Switch.  Handheld gamers are clearly choosing it as the successor to the 3DS, but home gamers are also choosing it as their next gen system.  Few will be buying the PS5.  So, when the Japanese look for the successor to the PS4, they choose the Switch.  The successor to the PS4 launched in 2017, and the result is that it cut off the tail to the PS4 in Japan.

If Sony wants to reverse this trend and really be competitive against Nintendo in Japan, it has to start in house. They can’t just rely on throwing huge money on exclusive deals forever, timed or not. Square Enix, Capcom, Sega/Atlus, and all the other Japanese 3rd parties may have other plans. And while I’m sure they’ll continue to develop their games for PlayStation, they can’t just ignore Nintendo and the Switch forever. More specifically, they can’t keep looking for excuses to justify to their investors why they are continuing to ignore such a profitable platform that has proven to he a grand slam not just in Japan, but worldwide as well. Investors will see that as leaving money on the table, which it is, and start to lose faith.

Sooner or later, mainline entries for all those franchises that have been almost exclusively PlayStation for a while such as Final Fantasy, Kingdom Hearts, and more recently, Persona, are going to come to the Switch as well. And if this downward trend continues when that happens, it’s going to look even more bleak for PlayStation in Japan, and it may even start to have some trickle effects worldwide. For example: If a sizable, considerable chunk of the Japanese audience chooses the NS/NS2 version of Persona 6, Next Gen Monater Hunter, or Final Fantasy XVI, over the PS5 version, who’s to say the same won’t hold true for the Western audience? So those are lost hardware sales, lost revenue, and lost profits, right out of Sony’s pockets that are going straight into Nintendo’s.

If they don’t want that to happen, which I’m sure they don’t. They have to take matters into their own hands and start to seriously develop, appeal, cater, and market with the Japanese audience in mind. Ghosts of Tsushima is a step in the right direction. They need more of that, much more.

If Sony get desperate they might try to moneyhat Dragon Quest or something, in fact it wouldn't surprise me if that happens, but if an exclusive (in Japan) mainline Monster Hunter couldn't bring PS4 up to PS3 levels, I doubt even DQ would dramatically alter PS5's fortunes.

There is no " reverse" in any way in Japan. Sony did everything and a litte bit more what they could in Japan - Death Stranding, Ghost of Tsushima, Nioh,Bloodborne, Gravity Rush,VR,Nier, tons of other JP games. Japan is done with classic home consoles, period.

Other JP publishers shall port their games to the Switch as well, but there are 2 ifs - sales, what is not great for 3rd party on Switch, and optimalization cost.

Aint gonna happen - Sony is - mostly-not doing " take it away" moneyhatting.

The other thing Sony could do is try their hand at a hybrid console, but that won’t happen without some major sacrifices in some areas.

I really don't see how PS4 is going to win on this comparison.  Switch is already 3m+ ahead and it has its biggest holiday coming in just a few months.  It's about to leave the PS4 in the dust.