The fanbase is full of people who cannot accept change and love to complain.
I definitely agree that Final Fantasy XIII was the catalyst for how jaded people have become today. The game was initially met with general praise. People changed their tune a couple of months later though. That's not to say the game isn't flawed and that one could have a legitimate discussion about its lack of exploration and such, but the way people went from at least thinking it was good to repeating how awful they thought it was ad nauseam (because everything is either black or white on the Internet) - it really felt like people just wanted to be negative for the sake of being negative. Misery loves company, as they say.
Ever since then the fanbase has been plagued by naysayers and cynics who think the franchise is dead, that it'll never be good again. They find various strawmen to support this argument, like "they need to get Sakaguchi back!" and "it's Enix's fault, they should never have merged!". Ironically, the merger happened because Sakaguchi almost ran the company into the ground with The Spirits Within. People put him on a pedestal, which they shouldn't. Yes, he created the series but mostly worked on the first five games - all of which basically recycled the same theme of crystals and warriors of light in some form. It has since grown so big it can no longer be credited to one single person. Kitase and Nomura certainly had a bigger role in creating VII than him, yet people think he should be brought back for the remake. Why? What good would that do? He has changed, too. Just like everyone else. This is a man who is now fond of making mobile games, something Square Enix gets a lot of flack for doing. It is also a man who has failed to produce anything noteworthy during his time at Mistwalker.
As for the complaints regarding Final Fantasy XV, they're all ridiculous.
Yes, it's an action RPG. So what? People who think that means button mashing and no strategy honestly don't know what they're talking about. But some fans are hung up on the idea that it has to be turn-based to be Final Fantasy. Oddly enough, Final Fantasy XIII used a variant of the ATB system. But I digress. ATB isn't proper turn-based either. Yes, you can set it to "wait", making it... not ATB... which many people like to do (which sort of defeats the purpose in praising that system). I forgot who it was, but the person responsible for the ATB system said in an interview that the original plan was to make Final fantasy IV an action RPG with real-time battles but they didn't want to risk alienating the small but growing fanbase the series had at the time so they created ATB as a compromise. In short, the plan was always to gradually sway towards more action based combat. If you look at other games in the genre, most of them have already made the transition. Turn-based combat in this day and age is archaic, unless it's an SRPG.
Yes, it has four male leads. So what? Bigots say it's gay, but then again they also said the same thing about a certain game with three female leads. "Feminists" say it's sexist, yet ignore the fact that Final Fantasy is known for its many strong female characters and that the most recent games star Yuna, Ashe and Lightning. For the record, there was literally equal representation in Final Fantasy XIII. That clearly didn't matter at the end of the day, did it?
They look like a boyband? Just because they're four boys, right? They can't just be a regular group of friends who happens to be all male? Or is it because they're all dressed in black? You know, the reason for that is because they're supposed to be similar to the yakuza. Maybe it's because a couple of them have "anime hair"? Whoa, if you really think that - are you really a long-time fan of the series? Or do you just want to play it because it's a big AAA release with great graphics and you normally play shooters? Final Fantasy has always featured androgynous characters, usually far more so than anything we've seen in this game so far. The irony is that most of the characters we've seen so far don't fall into Nomura's usual "belts and buckles" with ridiculous hair category. Noctis looks very Japanese indeed, but Gladiolus for example looks more Italian. Even Prompto no longer looks Japanese. He looks like a western guy who's cosplaying. In general, most other characters seem to have more in common with Auron than Squall or Tidus.
Last but not least we've got the stupid complaints about how it no longer looks like it did in the original trailers. First of all, we've barely seen anything from this game. Episode Duscae is basically the equivalent of leaving Midgar and exploring some before entering Kalm, albeit far more advanced in terms of what you can do. The few recent trailers we've got that involves some plot, they all seem rather dark. The only thing that's changed based on what we've seen is the combat system, which now uses something that resembles gambits for the AI controlled characters and the action has been toned down to make the gameplay more strategic. Also, Stella has transformed into Luna. I doubt they changed the entire plot. As far as we know, it's just an aesthetic change.
But haters gonna hate. Some people have unfortunately already decided the game is going to suck without even having tried the demo. Still, that doesn't mean they're going to shut up about it. They will continue to spam message boards with their negativity.