I'm no supporter of some of FR's practices, but they have shaken up the airline business model substantially by progressively switching fixed costs into variable, particularly wages. This means that 2 out of 3 main costs (fuel and wages) are truly variable and related to flying hours. This leaves the capital cost of the aircraft as the only significant fixed cost. If that cost is covered in the summer period, it only makes sense to fly the planes if the revenue then covers all the variable costs plus the xtra downgrade in aircraft value through increased cycles. Most businessess are seasonal in one form or another, and successful ones adjust their cost models to suit. Until ryanair came along, there was a general assumption (and still is in some quarters) that airline business seasonality was just on the demand side.