They'll 100% do a 3rd park eventually, just not right away. Which is understandable. Don't wanna do what WDW did, where it sometimes feels like they can't handle their massive scale. DL and DCA have some great areas where they can expand first. They first have to tackle the issue of parking, which has been a nightmare, as well as other logistics.
I predict that we'll see real rumblings of a 3rd park AFTER:
1. They fix and expand Tomorrowland
2. Fix Hollywoodland
3. Expand Fantasyland (Frozen, etc)
4. They add more Star Wars and Marvel
5. They add one or two more hotels
6. Acquire GardenWalk
7. Add more parking
8. Figure out a convenient way between the Esplanade and the 3rd park.
So basically, a 3rd park isn't in the plans right NOW, as there are other plans for the existing two parks. But there will 100% be a 3rd park in the future.