For me, I feel like the post FFX games do have issues, but I don't think I'd ever call any of them bad. For the most part, I'd say that after FFX is when Square started really messing around with the series seeing how it can keep it relevant to varying degrees of success.
FFXI appeals to a very specific audience, even among MMO fans and it refuses to change that. While the game certainly has a lot I could criticize it for I have to admire their dedication to their players. I feel like Squeenix really wanted this game to be bigger than it was (1.2 million at peak), hopefully putting themselves into a Word of Warcraft scenario where the MMO was so popular that they didn't need to make another single player game ever again. Obviously that didn't work out.
FFXII is more or less an evolution of XI's game play in a single player setting. IMO it's probably one of the most underrated games in the entire series, next to FFV. The only real problem this game had was it's lackluster story and characters, but for me personally I'd say that the game play more than made up for it. Granted, there was also the love-it-or-hate-it licence board and gambit system to consider, but I didn't really mind them.
FFXIII is not a bad game. I wouldn't even call it the worst Final Fantasy, that dishonor goes to Final Fantasy II. And I certainly wouldn't call it the worst JPRG I've played, that would Quest 64. It is flawed, yes. The battle system's attempts to be unique instead end up making many of the battles feel samey and the story really lacks focus despite how much they throw around that word. That being said, I think the Crystaltorium was an interesting evolution of the sphere grid, the paradigm system was a neat concept and I actually like a lot of the characters.
Now we get to FFXIV, or rather the relaunch of it, ARR. As a long time MMO fan I can confirm that yes, it is good, if a little confused on how solo oriented it wants to be. That being said, it's a good case for why the subscription model is healthier than free to play and there's a reason why it's one of the fastest growing MMOs on the market right now.
So yeah, as I said after FFX Squeenix really started experimenting with the series. That being said, Final Fantasy has always been about pushing boundaries and trying new things. Remember the time FFVIII asked "What if you could equip summons and spells?" Or how about when FFIV asked, "What if battles existed in a realm between real time and turn based?" The fact is that there's always been weak Final Fantasies and strong ones. I think with a lot of the post FFX game they did go a bit far in their experimentation, but as @
Minato
Squeenix is taking fan feedback on their games and are actively working to make the best games that they can.
IMO half of the reason where there's a census that FF has "fallen" is more due to the general backlash against Japanese media that started in the late 2000s and the general gaming community favoring western RPGs over Japanese ones right now.