FF15 sold 10 million copies.
After that the user-base was essentially split between 3 different games.
FF14 Online (released later & in proper form on PS4)
Final Fantasy 7 Remakes
Final Fantasy 16
So these kinda things do leave an impact on collective sales.
It’s not like Final Fantasy gained millions of more fanbase overnight after the release of 15.
So a general split will reflect in some form.