The graph is for achievements number of achievements gained by players in specific time periods, how many players make their accounts private? How many have most achievements? There is so much wrong with this data you can not draw accurate conclusions people in this thread can't even read it right.
I will again copy paste this statement by the person who made that graph from the same website.
Although the recent decline is worrisome, if you look at the overall number, you can see that the active character population is basically increasing, albeit in waves.
https://activeplayer.io/final-fantasy-xiv/