I've already responded to that claim a few times, but here's a quick summary:
-FFXIV has finished its main storyline. Games naturally will see a portion of their population leave after that point. It just happened with Destiny 2. It happened with WoW years ago when the Lich King expansion ended. They had an active playerbase at the time of ~12 million. It cratered right after when Cata released, and it recently took two solid expansions in a row just to get back up to ~2 million (meaning over 80% of its one-time playerbase has left). It'll naturally happen to some degree with FFXIV, too. Game studios end up in a lose-lose situation...they'll get trashed if they put a game into maintenance mode at the height of its popularity, and if they keep going instead, it'll never be "as good" as the culmination of the main story. DT is basically the new ARR. It's entirely possible that FFXIV will have another major hit of an expansion a little ways down the road - the question will be if the naysayers bring themselves to acknowledge that it took installments like DT laying the foundation for that future one to really fly.
-The game also hasn't lost anywhere near the portion of its "intended playerbase" as the negative echo chamber on the forums would lead you to believe. If you want the best data, just look in-game. I'll point out again that Hunt Trains on Aether started DT with around 200 people in the big trains, and I'm still estimating ~125-150 people in them now (harder to tell with no instances currently in play, but based on how quickly things melt compared to when there were instances, I think it's a reliable estimate). That's nothing out of the ordinary compared to the usual cycles that happen over the course of an expansion. Same goes with queue times when I do WT and queue up for very specific dungeons and raids that aren't the "typical" ones people run - queues are still popping within a few minutes like they always have. I think the "negative" or doomer viewpoint has been amplified more in DT than usual because there's a particular portion of the gamer population that tends to be more vocal on forums, reddit, etc., and they are most certainly not the intended audience for DT. But just because those voices are louder doesn't mean they're more numerous.
I think this is huge. The reality is that it's physically impossible for developers to create content anywhere near the rate players will consume it. A good MMORPG instead provides players with the tools and the spaces to make their own fun, and I believe FFXIV excels at that. It's a role-playing game; it's expected that you use your own imagination and the tools available to create your own experiences with your friends or even just within mini-communities of like-minded people.Just because the MSQ doesn't have an explicit multiplayer mode doesn't mean you can't make your own fun.
I'm afraid people have become too dependent on the game holding their hand for everything. I treat this game like a sandbox to find my fun in and I've never been bored so far.