I don't see why it (= PS5) wouldn't continue the downward sales of dedicated home consoles. Mobile gaming is growing at an excessive rate in Japan.
True, I was meaning more in terms of Playstation hardware specifically though; if (and that's still an if) PS4 sells less than PS3 lifetime in Japan, will PS5 sell even less again, or is there some way it can buck the trend.
If PS5 doesn't put up a stronger fight than PS4, Switch may never see a serious challenge to its dominance there in the dedicated gaming space, at least not until its own replacement.
With bold, I meant to say that I don't see why PS5 wouldn't continue the downward sales trend. PS4 literally had everything. A mainline Monster Hunter, Dragon Quest, Final Fantasy, Metal Gear Solid, Resident Evil, Persona, Nier, etc. Considering the ongoing growth of mobile gaming, I think that the conditions for PS5 would have to be even better for it to not sell less than PS4 and I don't see that happening. At least, a lot of Japanese developers manage to find success in the West, so there should be a good chance for PS5 to continue the strong Japanese support.
So yeah, Switch and other Nintendo consoles most likely won't ever see a serious challenge to its dominance in Japan in the dedicated gaming space, at least not from Sony.