If I could give 10 Likes to your post, I would. Spectacular piece of writing.
This is something I've found myself talking about to people as well, explaining what it is/was about FFXI's world design that made it feel so much more believable and "real" than most any other MMO out there I've played. Among other reasons (a sense of history, each with its own "personality", etc), the way they're designed to provide content across several level ranges, and how the mob levels (and other design elements - such as 3 Mage gate in the Ruins) keep parts of the map off-limits are key to what made XI's world so outstanding. Some area that was just beyond reach "for now"; a reason to return to it later.
It's awesome reading similar things to what I've said many times being spoken by someone else. It's also heartening to know that I now know of at least one other person who pays that much attention to such things as the nuances of world design while playing something lol.
I'm also an aspiring game designer and am, in fact, in the pre-production and prototyping stage of working on a game project. SE's approach to world design in FFXI is the cornerstone of how I'm approaching it in my own project. It plays into a sort of "pet theory" of mine on world design in games in general, but MMOs in particular.
Basically, I find MMOs fall into one of two general categories, one being "world-centric design" and the other being "game-centric design". There are many examples and illustrations I could give between the two, but in a nutshell, with game-centric design, the world exists "for you", while in world-centric design, the world exists "despite you". FFXI (and other MMOs like Lineage 2 (pre GoD) and Asheron's Call 2 (RIP)) were very much world-centric designs. Other MMOs like WoW, TERA and such are much more game-centric in their designs.
To give a simple illustrative example, consider the OP's description of the Ruins in Sarutabaruta, about how you go farther and farther in and find that the mob levels keep increasing, but that there's also ever-more to discover in there - going as far as Toraimorai Canal and Full Moon Fountain when you get to the core. That's an example of a world-design where the world exists despite you, as the player. That dungeon isn't there merely as a leveling mechanism, or the focus of a series of quests from a specific quest hub. It exists because it's a part of Vana'diel, with a history, an identity and an ecology all its own. It's there because it's there, and that aspect of it helps to give the world much more believability and authenticity, in my opinion. You'll have reasons to go in there at level 10, and you'll have reasons to go in there at level 70+. It never becomes obsolete. You never "out-level" it. The same can be said of many other areas across Vana'diel.
Now let's look at the other side, game-centric design. Let's take WoW as an example. You start off in, say, Teldrassil as a Night Elf. You start off in a smallish quest-hub area. Everything in that little closed-off corner of the map exists "for you". Every mob, nook, cranny and cave has a quest associated with it. And when you've exhausted all the quests? You're pushed on down the road to the next quest hub. You will never need to go back to that starting area. There's nothing more there for you. There are no higher level, tougher enemies lying in wait in some corner of the map, or a dungeon that you couldn't get to because your level was too low. Other than for personal reasons, or perhaps an "exploration achievement", it's obsolete.
So now you're in this new little quest hub, where everything in the surrounding area, once again, exists "for you". Every mob, nook, cranny and cave is tied in to some quest you're going to receive from a NPC in that hub. And once you're done? You're moved on down the road to the next quest hub, and this one becomes obsolete.
And so on, and so on and so on, right on up the line.
Progress in WoW and its ilk seldom has a rearview mirror, nor does it have a reverse gear. You are pushed ever forward, leaving in your wake a growing series of out-leveled and obsolesced content - entire zones worth. The world holds no fascination or meaning because you're continuously pushed right through it. Locations hold no magic or memory because they're merely quest hubs, one of dozens you'll be guided through on the way to level cap. Every area and its associated quest hub is contrived and designed "for you", with the sole purpose of "helping you level up". Rather than adventuring through a persisting world of which you're a part, you're just a tourist passing through.
That's an example of game-centric design and it's the complete opposite of the approach SE took with FFXI.
So, I'm with ya, OP, on this. I hope SE brought some of that world-design magic to 2.0 that they had in XI.