Looking at the responses, if this was about Sony having a "monopoly" then Nintendo would be criticized here for not having a platform that is appealing to the Japanese market rather than the "<insert platform here> dominating is bad".
PS4 dominated Europe/Middle East/Asia against Xbox One and Wii U and that was hardly considered a bad thing. Any consumer with a brain knows that they'll buy a product that appeals to them, they're not going to go and buy a Wii U out of pity for it not selling well.
Regarding Sony in Japan, the writing was on the wall the moment they consolidated everything around the PS4 and dropped the Vita. Anyone that has followed the history knew where things were trending and them killing off the Vita made that transition quicker. Sony is the only competitor that decided that their console hardware is for playing the biggest games from the big publishers and the Vita lacked that from third parties and Sony wasn't going to put their weight behind Vita when PS4 was so successful.
We know from past articles written by Takashi Mochizuki that Sony was going to cater to the big publishers for PS5 and that Sony executives said that smaller developers will follow because they won't ignore the platform and yet... Reality doesn't follow that in Japan.
We've known for years based on sales data what kind of games sold on PS4 in Japan and even
demographic surveys that confirmed the active PS4 audience is centralised around male teens and adults. The software library on PS4 was no longer as broad as on PS2 to attract a wide variety of people by age and gender.
Meanwhile, what did Nintendo do with the Switch? They pushed relentlessly to make Switch appeal to anyone by not focusing on making games that would appeal to the typical male teen/adult audience. They also knew from their failure with the Wii U that content is king so they reached out to Japanese and Western developers to put their games on Switch because some third parties still viewed Nintendo platforms as being family focused that they felt their games wouldn't sell. Remember Bethesda? It took Nintendo reaching out to them to get their games on Switch that when Bethesda's games exceeded expectations they continued to support Switch, we wouldn't even have Doom Eternal on Switch if their games didn't sell. Mortal Kombat 11 is still most likely the best selling third party game on Switch with an M rating (see how it debuted on NPD).
The point being is that PS5 is going to be a niche console in Japan because it only appeals to a specific group of people because of the decisions that Sony made. The Japanese small developers that relied on the PlayStation ecosystem for so long have jumped ship to Nintendo because they could no longer sustain their business on PlayStation alone.
Nintendo dominating software charts in Japan now is no surprise when there are more Japanese third party developers supporting Switch than PS5 and Xbox Series X|S combined.