Yes those zones are truly huge but they are lacking quite the life. Especially underwater. You have a huge mass of land with a bit of plants everywhere and some monster sprinkled around it. It does not feel alive for me. You only have monsters that are out to kill you so how can these farmers survive this, if those monsters are even around the camp and are attacking you if you just walk on the normal path? Where are all the critters and non violent animals? Why are plant eating animals attacking us on sight? Where are the herds in the Azim steppe? Why is there no true actions between different kind of animals or monsters? I mean in the new diadem you will have non violent creatures that are attacked by meat eaters in fates. Why not have something like that in the zones too? Then I at least would feel like these are alive.
So they took their budget, make those big zones yet they dont feel alive and are barely used anyway. They could do way more with this, or just create smaller, denser zones like some of the ARR ones and take the time and money and create more new endgame stuff instead.
MSQ and job quests are done once and thus are not something you do repeatably. One can only do so much fates and hunts after so many years before it feels boring and repetitive. So its kinda bad to say that people are not willing to do anything if they have to do content that was already done in ARR..
You know in Rift you can gather artifacts that are lying all over the landscapes as small balls of light. Some are next to a path, some others are behind items, some are far off the path and much more. Its fun to explore the whole map to get these and they are not so easy to find since there are no hints where they are. (One could buy an ability that shows them for 30 minutes for real money but you are not forced to use that) Also you can stumble over a riddle in each zone which must be solved differently and grants you a chest with a nice price. At the same time you are able to find books from time to time in NPCs houses or in the zone, which you can take with you. They will tell you about the lore of the game, something that is quite lacking in a story driven game like FF14. You know instead of having all the lore information in a real life book, they could have been included as ingame books too which could be read in the game. This would have been another way to use the zones.
The only thing that is even near something like that in FF14 is the sightseeing log and that was made way easier since HW.