Square should just focus on making good games, not on appealing to 
any "current market trend" or "target demographic", because they'll wind up diluting their skills, entering oversaturated markets, and losing focus on their designs.  And sure, 
FFXV sold more than 
Persona 5.  But 
Persona 5 was lower budget, built by a smaller team, still has plenty of DLC that is continuing to make ATLUS money, and has developed the sort of rabid fanbase who will throw money at anything with the 
Persona name on it that Square used to have for 
Final Fantasy.  That era has passed for Square, and it won't come back until they rebuild a franchise with some consistency and history to it.