I'm not gonna try to argument that DT writing wasn't that bad. It was, maybe even worse than that. That said, I feel the biggest problem of FFXIV lies with the content.
EW did all it could to make the game dead. Lately, when I want to play XIV (like right now), I don't because basically, except 3 or 4 roulettes, there is nothing worth doing in the game. The feeling of wanting to play a game, and connecting to realise there is nothing you can do or leaving because you already did the few things you could do but still wanted to play is so frustrating. To me, it's the worst feeling a persistant game can give you. I'm all for the "not trying to keep you in at all cost", but I feel like a game like this one should have the possibility to stay in for as long as you want if you feel like it. Right now, you can't do that if you're not a fairly new player.
In the past, the role of keeping players that wanted play more was held by the relics and exploratory zone (I consider those as two things, because despite being related, exploratory zones, in their last iteration, had a lot more to offer than the relic). As perfectible as they were, exploratory zones gave a lot of side objectives/occupation (getting glams, farming for mount/haircuts, being decent leveling places, having their own side content - like the Ilkil farm or the medals in Bozja/Zadnor). We had a lot of discussions about those in the past, but most of them seemed to not put the light on why they were so important. This content was designed as engagement content, to keep you occupied between patches. It did it by giving you side objectives you could see yourself progress in at a decent pace (not like farming the pterodactyl or the bicolor gemstones mount). Changing the way relics work and removing exploratory zones would not have been such a problem if we were given another content that filled that role. (I would add it was one of the only content you could jump in/jump out as you wished, which is severly lacking in this game, but that's a discussion for another time).
Then came DT, and the horrible story killed any excitement, making the problem far more obvious. We had discussions about the relics and exploratory zones for 2 years, and SE doesn't seem to have understood that point, since it seems their calendar to give us content that fill that role hasn't changed. Our best guest it's gonna come in 7.25, in 7-8 month. With the excitement mostly dead, this is far too late. Which lead me to think they did not even understand why the removal of an exploratory zone and the change in relics were problematic. When you're ready to extend a lull that already lasted for 2 years for 10 more month, there is a big problem with your patch cycle planning.
I've been a mmo players for years, and I still don't understand how a professionnal team could forget so much about what the mmo genra and the mmo players. Sure at this point "classic" mmo players probably aren't the majority of their player base. But it's the people that make your game not dead between patches. The people that makes the population healthy enough all year long, and make possible for new players to begin anytime. It's no surprise the people that came during the WoW exodus mostly went back to WoW, because despite all its problem, WoW understand what a mmo is, and give you enough content to keep you occupied. XIV seem to have forgotten that.
I also realise this a problem only encountered by long time players/players with a lot of game time, yet I feel like trying to not bore those to death is as important as trying to get new customers or pleasing the raiders.
TL;DR
The reall FFXIV problem is the lack of shelf content, engagement content made to last a little by giving you side objectives you can see yourself progress in at a decent pace.