This is absolutely true. One thing I think they have learned (based on conversations I've had with some people here) is that, after gate prices, there's diminishing marginal returns on adding people to the parks - once you get to a certain point, revenue per guest starts to decline while costs go up. That's why they're limiting capacity still.
Only part of the story. Stuffing bodies in the door and eyeballing the gate admission has never been the main draw -- that's why they've happily severely discounted admission to extend stays. Admissions are really just 'opportunities', but something they can also monetize. The real meat is daily spend of each guest. Admissions are a pay once thing... and their value only goes down with upsells. SPENDING is where the best revenue growth opportunity is.
Adding capacity requires large capEx spending and becomes more overhead to maintain. While extracting more dollars per guest per day can be done much cheaper, or even 'for free'.
Attendance can drive grosses pretty linearly, but doesn't drive margins nearly the same. The park overhead is not linear, it has stepped minimums and then costs escalate as you approach extremes. So pushing attendance as a growth driver is not unlimited.. you actually want attendance to be in your 'sweet spot' and consistent... not just 'mawr mawr mawr'.
When you are overcrowding Revenue per guest starts to decline because of dissatisfaction and time constraints the guests have. Overcrowded parks are also more expensive to operate as it takes higher levels of staffing just to manage the excess.
This is why Disney's model has been to drive revenue growth through higher prices and optimization -- not primarily through more capacity.
Disney has focused on driving higher per guest spending and optimizing their existing capacity.
1) Raising prices
2) Adding monetization
3) Adding upsells
4) optimizing their costs by reducing overhead (run only what is necessary)
5) shaping demand by moving guest loads to other periods
6) improving optimization of existing capacity by keeping guests distributed across parks to Disney's benefit. (They aren't going to NOT open one of the parks.. so its better to keep people trapped longer to get that park capacity utilized more)
Disney won't really see capacity as the 'needed' thing until the two strategies above are saturated and nothing more can be squeezed out.