Actually, the situation you just spelled out is the law of demand: a higher price for a good or service, other things being equal, leads people to demand a smaller quantity of that good or service. (and vice versa) That's why
DCL is finding they are having to discount their prices in their "off" season now; people are no longer paying the higher price.
As to the OP's original point, all cruise lines that cater to families (yes, including Carnival) raise their prices during school break periods. The number of people available for cruising during those times increases; that is a shift factor of demand (which price is NOT) and causes a shift to the right of the demand curve.
So as to not sound like an ECON textbook:
All else being equal, a change in price will cause inverse shifts along the demand curve. Prices go up, demand goes down.
EXCLUDING PRICE, changes in:: 1-prices of related goods or services, 2-income, 3-number of people, 4-tastes and preferences, 5-expectations (future prices or availability) will all cause shifts left or right in the entire demand curve. Which explains why cruise lines (and other travel industry establishments) can actually charge MORE during summer and still fill their ships.
Oops, that still sounded like an ECON text book.
Having not sailed the Allure I can't specifically answer the comparison question but I'd save the 2 grand and go RCI.