The problem with radical jumps in prices is that one does so at the risk of alienating the consumers. Even now, Disney offers discounts (or free meal plans) to try to get more attendance. If the price jumps too radically, then comsumers may come to believe that the product isn't for them (when was the last time you priced a DeLorean?). At that point, Disney will have lost that consumer forever. When the economy turns (and it will) Disney would have a hard time re-establishing a customer base. Drastic spikes in price might create the current desired outcome (less crowds) but it does so by risking the long-term viability of the parks.
As for the argument that price spikes might actually increase attendance, this is only true for die-hard Disney fans. For more common people, they will just determine that Disney is out of their price range. Disney vacations (if ever taken) will be one-and-done events. The strategy also opens up a real possibilty for someplace else to assume the role of the "family park." Disney spends a lot of money to reinforce the idea that it is the ideal location for families. Yet, if their prices are out-of-reach for most families, then no amount of advertising will convince otherwise.