At the risk of getting repetitive, I firmly believe that FFXV is going for a "content as its own reward" style.
I'm in love with the story of most Souls games, it's one of the main draws for me, but let's not kid ourselves, the main reason why people go into the Painted World of Ariamis is because they want to experience the content. They want to explore the level, fight the unique enemies, and face the unique boss fight.
By contrast, in Witcher 3, if you judge a cave by the same standards you judge a level in Dark Souls, it'll be extremely shallow. It's just a tiny dark place you walk around, press X a few times, maybe swim from one point to another, kill the thing and you're done. There's not a lot of complexity in the content itself. That's not to say there is no depth, of course, it just comes in a different way. You're on your way to your objective, whatever it is, and you notice a trail of blood going into a cave. Just outside the cave, there are a bunch of corpses wearing nilfgaardian armor. You want to know where they came from, where they were headed to, and most importantly, what the hell hapened to them. The story context and world building serves as motivation and reward for what would otherwise be a very uninvolving cave exploration.
Coming back to Final Fantasy XV, if you watch the PAX presentation, you'll see that there is little to no context behind that dungeon. It's pretty much:
NPC just outside the dungeon: "Heeey it's woooooooooooooooo so dangerous inside woolololo"
Noctis: "aight, shit, sounds fun, let's go, owooooo"
But then you enter the dungeon, and there's a fucking elevator that can drop you in different floors, from how layered that dungeon is, and you have to unlock those shortcuts through exploration. The level design is much more intricate and tricky than what you'd find in Witcher 3 (even if not as tight as what you'd expect out of a Souls level, but Miyazaki is the best level designer in the business, imo), the enemy placement and behavior is scripted in a very deliberate way to keep you engaged, and it might even surprise you in some moments. It culminates in a "mini-boss" of sorts, and maybe there's even more to it after that, but the dude didn't beat the enemy. Given the enemy's level, it's even designed for your covenience if you choose to run away and go back later, after you've leveled up a bit, using the elevator shortcuts.
Encounters like that goblin locking the door in front of you, that you later unlock from the other side while exploring, the surprise cart attack and stuff like that is all designed and placed in a way to be engaging and rewarding on its own.
To mention the Adamantoise example again, even if there's not a cool story around that encounter, and there probably isn't, I don't really care, I REALLY want to fight that thing. Being familiar with the game's combat makes me crave for that fight, it's not as much about curiosity and solving a mystery as it is just the pure satisfaction from having a battle on such a scale in real time as part of an explorable world, using those elaborate combat mechanics.
It's pretty clear which approach I tend to prefer, but I do love Wild Hunt, I'm having a blast playing it, but I don't see not taking the same approach to open world design and exploration as a sin. Each game does it in a way to fit its strengths better. I should also remind you that, while a lot of people called bullshit on the "having a lot of empty land to traverse is important to transmit the road trip feel" thing Tabata said way back, when the press got to play it for a while, jschreier (iirc) even mentioned as a negative that Lestallum was too close to their current location, and that it should be a longer drive based on what they were saying. They went out of their way to make things big just to make your travel time seem believable, and the criticism Schreier had about it was that it wasn't long enough. Imagine if they didn't do it.