I don't agree with that. PS4 is almost at 10 million in Japan despite being a home console. So home console won't disappear in Japan.
Plus Final Fantasy VII Remake will be soon out so PS4 will be at 115 million units sold in Japan in matter of days so that's all good.
ps4 mostly competed with the wii U (a harder fight than what most people think thanks to INSANE staying power of splatoon, ps4 only surpassed WiiU around when the NSW was announced officially), while the 3DS was absolutly CREAMING the vita despite japan being the vita's best reigion by far. so basically PS4 was alone in its target market, similarly with the 3ds.
then the switch came along and threw a wrench in the ps4's long- term selling power in japan.
ps4 is mostly still coasting along its pre-2018 install base.
the PS5's the first playstation since the first where its future isn't guarenteed in japan, and that might be enough for japanese devs to shy away from developing up to ps5 levels of power, its a similar situation to when the ds and psp came along and console gaming never recoverd its previous status casue the psp/ds was "powerful enough" for most devs, exept that now the switch has enough juice to run ANY type of game as long as you restrain and optimize.