I think the real danger for Sony if the PS5 doesn't sell well at all in Japan is losing Japanese exclusives that sell in Japan... DQ, KH, FF, MH (at least in an exclusive to Japan sense), Persona, and Yakuza, to name a few from the top of my head. I think the transition to the PS5, leaves the door wide open for developers to port games to the Switch as well, if there isn't a strong Sony presence in Japan, developers who make games for the Japanese market, might be forced to bring their games to the Switch.
We will see it coming for sure, Atlus will probably be the first company to bring something over, also MH coming to the Switch for stronger Japanese sales makes a lot of sense, but we will have to wait and see if either of those things happen, but if so, we could start to see a few more games from those series I listed above make their way to the Switch.
What if Nintendo Switch is an immortal platform?: Losing the Japanese market could be a very big deal for Sony, and what if the Switch '2' isn't replacing the Switch, but extending the platform for another 5+ years, what if Nintendo's plan with the platform is to eventually offer streaming on older hardware and to keep the platform alive for 15 to 20 years like they did with the Gameboy, Gamecube hardware, DS/3DS... What if their goal is simply to have a platform that doesn't expire and they just keep selling Switch units throughout Japan and the world, right now they have over 52 Million Switch units in the world, by the time next gen arrives, it could be 70 Million+...