Creating a living breathing world, certainly sounds like a great idea. Especially to marketing folks. If that world is also an already known entity, even better.
I'm perhaps in two minds. Building these believable worlds you can escape to, is exactly what the Disney parks have been about, especially the Disneyland/Magic Kingdom parks, where you can feel fully immersed within a jungle adventure, the wild west, or a fairy tale. So these new developments with their insane budgets should really be exceptional versions of these areas, however after experiencing a couple, I came away with the feeling I had far more to do in the older lands than some of these newer ones.
Pandora is a great example of immense detail throughout, but not a great deal to do. Just two attractions, a shop, a restaurant, a photo-pass point and two 5 minute street shows (the shop being so small that when it rains it hits capacity quickly and cast members then have to ensure there is a path for those exiting Flight of Passage to make their way through and out). Compare that to any Fantasyland, or Discovery/Tomorrowland and there is quite a gap, regardless of how good those two attractions are.
Diagon Alley has a lot more to explore it felt, even if itself only has a single attraction, not counting the train. However, the attention to detail means that if you had no idea of the story, you could quite easily miss the non-descript entrance, and think that the King's Cross station is the whole lot. I didn't even find Knocturn Alley until maybe my fifth visit through, but perhaps that is a good thing, to keep finding new areas to explore?
Other parks have done it very well. Cars Land at Disney California Adventure still being one of the best immersive experiences, with a large attraction and several smaller ones, along with shops, food places and street entertainment. Rookburgh at Phantasialand is another, perhaps benefiting from being deliberately smaller (or at least marketed as being smaller) from conception, compared to the massive things Pandora, Galaxies Edge and Diagon Alley are meant to be.
Immersion is great. But perhaps some of these new developments look really pretty, but simply don't have enough in them with respect to the scale of the development. And I think that's my issue. If Disney are going to spend $1bn on a single land, it should open with more than 1 attraction. Especially given that a new Disneyland style park is typically $5bn.