No games aside from minecraft and similar clones are actually fully seamless. Minecraft and similar are procedurally generated (and often awful, if not unrealistic looking) and these games are very-very-very empty other than a few choice prefabricated biomes that appear in places.
Games that have lore and storylines actually need to have "baked" scenery, at least as far as the places the storyline takes you. However the need to do gathering/hunting for crafting is another reason why zones are fixed a certain way.
I'd love to see a game correctly use procedural landscapes (Landmark was actually a non-ugly minecraft if you believe it or not) but it was also just a bunch if tiny instanced procedural, morphable, self-healing, landscapes rather than anything with lore potential. Also it turns out that letting users create buildings/dungeons, just has them create a lot of dead-ends for people to get stuck in.
The thing in FFXIV V1.0 was that Ul'dah was one map. It wasn't a north and south section. Likewise with Gridania. You frequently end up having to do quests that cross the zone boundary, and even if you play the game from SSD, the fact that you have to wait several seconds to cross it, and then have to cross it again later feels jarring. Like, for some reason they decided to put the crafting sections in one half of the map and the Central Aetheryte and GC building in the other half. It's especially annoying in some quests where you repeatedly cross the zone boundary. It might only take a few seconds for me, but someone playing on a laptop probably sits there for 30 seconds.
I could see the developers try to restore the single-map versions of the cities, as I don't really see any point, to how they're broken up. There's never 200 people standing in one spot except when new content places a new NPC (as was the case with RDM) and even that didn't matter because it was the solo-instance that bottle necked it.