Well - I'm going to disagree with this. I think Disney is doing pretty good to manage to find 5,000 seats in an already existing area. Fantasmic was built from scratch and is only 1,900 more seats. Disney can't build these areas based on expected crowds on opening night. If they built a 15,000 seat area then 2 years from now it would be playing to half empty seating.
What they will have to do in manage the crowds. Whether through dining packages or FP+, this is what they did in DCA with World of Color and they'll have to do the same thing here. Having 2 or 3 shows a night at first might also help significantly, but yes people will be disappointed and have to be turned away.