Has there been any main FF that was anything other than a commercial success? FFXV may have a detractors but it was considered to be a return to form by a large amount of people. You don't just abandon the series that puts you on the map, and if Square Enix don't have Final Fantasy, with all due respect to their other franchises, they're going to suffer massively, whether you feel that is justified or not. I don't think SE will decide to just stop doing the thing that causes their shares to go up, especially after FFXV got the best critic / user ratings since... hmm. Let's put this into perspective.
Year - Game - Critic Rating - User Rating (courtesy of Metacritic)
2018 - FFXII:ZA - 86 - 7.7 (PS4)
2018 - FFXIV:SB - 87 - 7.1 (PC)
2016 - FFXV - 81 - 7.5 (PS4)
2016 - WoFF - 77 - 8.1 (PS4)
2015 - Type-0 - 72 - 6.5 (PS4)
2015 - FFXIV:HW - 86 - 7.8 (PC)
2014 - LR:FFXIII - 66 - 6.0
2013 - FFXIV:ARR - 83 - 6.8 (PC)
2012 - FFXIII-2 - 79 - 6.6
2010 - FFXIV - 49 - 3.8 (PC)
2010 - FFXIII - 83 - 7.2 (PS3)
2006 - FFXII - 92 - 7.6
2001 - FFX - 92 - 8.8
2000 - FFIX - 94 - 8.9
1999 - FFVIII - 90 - 8.6
1997 - FFVII - 92 - 9.2
FFXIV in small because I'm not really paying attention to MMOs in this list, but thrown in to show signs of improvements over time in the overall history of SE FF releases.
Also bolded the 'dark days' games (from my personal perspective).
I look at user scores. So basically their latest three single player releases are the highest ratings they've had since FFXII, and FFXV is very close to FFXII in user reception. It doesn't match the golden age where VII-X kicked in, but those scores aren't reasonable expectations in a modern game - only a few per year get that kind of scoring these days. 8s and 9s are just that rare in all genres, let along RPGs. Overall I think SE have no reason to stop now - they are on the up, not the down, when it comes to FF sales and critic/user reception.
And let's face it, critic reception always gets judged by the last game in the series and how the users reacted to it (note: this is why FFXIII got a higher score than FFXV, imho). So I'd say FFXV did pretty damned well considering on that front!
There's a lot of cash left in this franchise and SE would be stupid to stop now when they are returning to form and favour with the majority of players. FFXVI is all but guaranteed.