I believe FF will never experience big growth like other franchises
It's a game that heavily changes the gameplay every entry. People always get dissatisfied with something. I'm talking with some friends who liked FF XV (it was their only FF) and are not liking XVI because they can't control party members and exploration is limited compared with XV open spaces
It's hard to build a core fandom when some games are so different from others. Of course you have people like me who are fans exactly because we don't know what we're getting, but weirdos like are are the minority. Most people want their games to be exactly the same with some iterative evolutions
Even if XVI experience great word of mouth and end selling 15 million (won't happen, but let's pretend for the sake of the argument) I believe this will not lead to a big increase in sales for the next entry, specially if they change everything again
