I will defend Square Enix for a moment.
I don't think SE has some crazy favouritism towards Playstation. They never considered releasing a mainline FF on Switch purely because it's specs do not serve their ambitions for the mainline games, same reason why they left Nintendo during the N64 era. FFXV launched right around the time of Switch & was multiplatform but they never invested or considered a port, they simply do not have interest in porting down their games... it would have broken the vision they have for that game and every other mainline FF.
In Squares mind Playstation was the only real console option for Japan. Xbox multiplat sales potential is highly overestimated (Has any JRPG sold more than 1m globally on Xbox?) and I think PC is only just proving itself in terms of being a major market for the genre, but we still don't have any concrete numbers there. Switch 2 will be a turning point in my mind as Nintendo will finally release hardware which is both popular and capable enough to run modern games at decent settings.
Lastly Final Fantasy's decline is related to more than just platform. I mean Monster Hunter Worlds' Japanese retail launch more than doubled Final Fantasy's VII R & XV's sales. And all those (MHW) sales were from Playstation. The FF franchise isn't capturing Japanese audiences like it used to, regardless of platform.







