Why do you think that Japanese companies are going to stop putting their games on Playstation? I haven't seen that any indication that big companies like Square-Enix, Sega, Koei-Tecmo, Bandai Namco or Capcom are not going to release their games on PS5.
I don't think they stop, I'm saying that if they ever did, the sales would have been rock bottom as Sony themselves does nothing to support sales in the market. The very low software sales attest to both this and that quite a lot of consoles have been scalped out of the country.
The PS4 didn't sell because of Sony's support, it sold because of third-parties. The only Japanese first-party game that sold really well in Japan was Grand Turismo Sport and even that only sold 280k, less then Horizon that sold 290k, Spiderman which sold 307k and Ghost of Tsushima which sold 439k. If PS5 gets just as much third-party support as the PS4 did then I don't see any reason as to why the PS5 won't do just as well.
Best selling PS4 games in Japan: