The reason XI is still running and is popular so long after creation, in spite of strong competition from other games with far bigger budgets is because it fulfills a niche that players want and that these bigger games cannot ever provide.
XI has long hard goals that appeal to progression junkies, difficult and somewhat unforgiving gameplay, region free servers, less spammy rotation based combat, a lack of UI community features that force players into community interaction, high rewards for progression that holds you above others, is faithful to the world / lore, hands off moderation over community interactions, impresses on players that this is a world and not a video game.
The reason the game is still running is because it tries hard to stay true to what its always been, and should always try to do that. Many of the things above would be considered gross negatives in modern mmorpgs, but a large niche of players do not see it that way (and when I say they don't see it that way, I mean they are here).
Even if many of us complain about the "grind" or the difficulty, the drop rates or other things in the moment, we still login tomorrow because even if we moan about it we kind of like it for some reason or another. The day I stop logging in, will be the day I have nothing left to work towards and little else.
To use an analogy, FFXI is like an extra hot spicy 9 alarm chicken wings seller on the side of the road next to a sea of big chain fried chicken sellers.
XI will always have a place as long as it remains true to what it is, and does not try to copy bigger games too much. If you try to copy the bigger games, you will fail because they already do that better than you ever can.