You, and they, are assuming that demographics are fixed things. "This appeals to millennials, this appeals to the next generation, etcetera".
There are several big, and rather obvious, problems with holding this position and also trying to appeal to everyone.
The first is that your game will lose coherence and direction, and suffer because of it. FFXV is a car driving simulator, an open world game, a fishing game, a Cup Noodle ad, etcetera. It's not an RPG, or an action game, but some unfocused amalgamation.
The second problem is that it will leave the company constantly chasing trends. Look at Boss Key Productions. They start making a game like Lawbreakers, to try to chase the Overwatch money. When they can't break into that niche, they abandon it and switch to Radical Heights, attempting to copy PLAYERUNKNOWNS BATTLEGROUNDS and Fortnite. Each time they try to go aim for a new audience, they'll bring less experience, and less innovation, and they'll wind up a pale shell that is laughable in the face of a market that will already be saturated (or, given how long it takes Square to actually MAKE games, already passed by completely).
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.