It doesn't seem surprising to me that if there is demand, but supply is capacity limited, it's hard to determine the full extent of the demand, and that when more capacity becomes available, the demand may exceed that as well.
If you build more lanes where the roads are generally uncongested, you won't see much 'induced demand', because the there wasn't much demand for travel that was waiting for less congestion.
If you build more lanes where the roads are generally congested, you will see the amount of time the roads are congested decrease when construction is done. However, most additional lanes are built in areas where there is strong population growth, leading to increased demand overtime anyway. For example, in southern california there was a project to add another lane to the I-405; when completed, the average duration of heavy congestion (rush hour) was reduced by several hours; but it's reasonable to expect that over time the congestion will increase again, just as it was before construction.
It's reasonable to expect the same to happen for theme parks. There are a lot of capacity constraints preventing people from attending a Disney park, and you see that in the high attendance. It's probably difficult to predict how many additional parks you could build and still have good attendance, but building one at a time seems reasonable; being a bit hard to get in isn't terrible for Disney's brand perception.