I agree that guests probably won't be staying longer, but that model isn't the whole story. We don't just have to think about average stay, we also have to think about capacity. Truth be told, Disney would much rather have lower ticket prices and more people at the parks. They would make much more money selling hotel rooms, F&B, and merch. Instead, because they don't have the capacity to serve as many people as they would like, they've substantially raised prices. But this model is not sustainable, and it's not in the best interest of shareholders to continue down this path for much longer. The Orlando market has grown substantially in terms of new visitors, but not much of that growth is attributable to WDW. There is a plethora of new potential customers on Disney's doorstep right now, and Disney isn't letting them in. The only reason the parks aren't at capacity everyday is because they've raised prices so much. A fifth gate would solve this by dramatically expanding park capacity. The reasoning behind a fifth gate is a lot different than the previous 4 parks, hence why there's been such a long gap. Relatively soon though, a fifth gate will be more than justified, it will be a necessity.
The reasoning is actually exactly the same as the cruise ship model.