One important thing to note is this translated statement from LB's blog entry that contains the graph:
Not all players make their character achievements public.Therefore, rather than looking at the absolute number of the latest active characters, we have been using the changes in the achievement information of the characters whose achievements have been made public as a reference for looking at changes over a long period of time.
For those of us who can't read Kanji, this is what the colors on the graph mean (translation courtesy of Google Translate - if you can read Kanji please feel free to correct the translations if needed).
Blue - New ("the number of characters whose first achievement was recorded during that period.")
Red - Continuation ("characters whose achievements were confirmed at that time, excluding new and final characters")
Green - Final ("the number of characters whose achievements after that period could not be confirmed.") (I interpret this to mean players that were active during the quarter but had no new achievements earned in the quarter as of the date the data was collected - I could be wrong about that)
I don't know if he collects the data only on the dats listed in his normal census or if he does additional collection on other dates. If it is coming strictly from the census dates, it explains the strange appearance of the 2nd QTR bar - the census for 2nd QTR would have been done on May 20th, 2 days before Patch 6.4 was released. The census prior to that was March 6th, just before patch 6.35 was released. It's not likely many veteran players would have earned new achievements between April 1 and May 20th because there was no new content with new achievements released during that period.
That there are fewer players active right now is known and it is normal for MMORPGs in the latter half of an expansion.
It is very possible that the game did peak with Endwalker's release and may be starting a slow decline similar to WoW.
So what? It's a 10 year old game, which is much longer than most games remain relevant. Think about the games you were eagerly playing 10 years ago (if you can even remember their names). Are you still playing all of them at least once a month? Probably not. Likely you got tired of them and moved onto other games then got bored with those other games and moved on again.
It doesn't matter how many other players are playing FFXIV. What matters is whether you personally are enjoying it right now. Some of us are, some of us aren't. Play according to how you feel about a game and not because of some information on a graph.
I definitely expect Dawntrail to be lower than Endwalker. Many of those who were "testing the waters" following the WoW content creators have stopped playing.
That's something that's going to cause problems when it comes to predicting if there is a specific trend - the WoW Exodus turned the normal number flow into chaos.