While Potter 2.0 will bring in crowds for Disney initially, you are correct about long-term impact. Comcast also shows no sign of stopping their bold construction. Kong might be completed by July/August of next year, KidZone is supposed to close this fall for a demolition of everything except Animal Actors and E.T. That opens summer 2016. Add the small additions over the next two years, the whole Citywalk remodel, the new waterpark, the 5th hotel, and whatever big 2017 addition is added... Disney's got Pandora and Disney Springs. IMO, that's too little over that amount of time. Star Wars Land, Pixar Place expansion, GMR & Everest refurbs, a Tomorrowland redo, and an overhaul of Epcot should have opened from 2010-2020 along with New Fantasyland and Pandora. A Frozen dark ride should also be fast-tracked in whatever space remains in Fantasyland (perhaps behind B&TB).
The more capacity and guest spending Universal gets, the lower attendance WDW's other 3 gates will receive. Magic Kingdom will always be a top-tier park due to its history and admittedly solid theming. However, by 2021, I see both Universal parks getting as many visitors as Epcot or even more. They could reach 12 million potentially. With 3 theme parks getting 10-13 million in guests and a water park that gets 2-4 million guests a year. You're near Disney's levels... not quite to 47 million but still.